Fix a crash caused by integer underflow

index is unsigned, and could be zero, so 'index--' will underflow and
round to 0xffffffff, and cause problem in a later call to Expand_Series

I believe the idea of

    if ((REBINT)index > i) index--;

is to move the index forward because gob being inserted were in PANE,
and they can't be there twice (Detach_Gob will remove them from the
current gob). So if they were not there, "index" shouldn't be changed.

This fixes a crash in the following script:
    REBOL []

    foo: make block! []
    for i 0 15 1 [
	    txt: #"a" + i
	    append foo make gob! reduce/no-set [text: to string! txt]
    ]

    g: make gob! []
    append g foo

    g/pane: next g/pane

With this stack trace:
zsx@touchsmart-arch:~/work/r3.git/make$ R3_ALWAYS_MALLOC=1 ./r3-view-linux test-insert-gob-crash.r
=================================================================
==24248==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60d00000c8a8 at pc 0x522243 bp 0x7fff91ba7a60 sp 0x7fff91ba7a50
WRITE of size 8 at 0x60d00000c8a8 thread T0
    #0 0x522242 in Insert_Gobs ../src/core/t-gob.c:230
    #1 0x5242d6 in Set_GOB_Var ../src/core/t-gob.c:421
    #2 0x5278ee in PD_Gob ../src/core/t-gob.c:713
    #3 0x414cda in Next_Path ../src/core/c-do.c:399
    #4 0x415c8c in Do_Path ../src/core/c-do.c:463
    #5 0x41a018 in Do_Next ../src/core/c-do.c:928
    #6 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #7 0x4a6eca in N_do ../src/core/n-control.c:524
    #8 0x43e267 in Do_Native ../src/core/c-function.c:289
    #9 0x419562 in Do_Next ../src/core/c-do.c:886
    #10 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #11 0x4a755e in N_either ../src/core/n-control.c:598
    #12 0x43e267 in Do_Native ../src/core/c-function.c:289
    #13 0x419562 in Do_Next ../src/core/c-do.c:886
    #14 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #15 0x4a755e in N_either ../src/core/n-control.c:598
    #16 0x43e267 in Do_Native ../src/core/c-function.c:289
    #17 0x419562 in Do_Next ../src/core/c-do.c:886
    #18 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #19 0x4a755e in N_either ../src/core/n-control.c:598
    #20 0x43e267 in Do_Native ../src/core/c-function.c:289
    #21 0x419562 in Do_Next ../src/core/c-do.c:886
    #22 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #23 0x43f8f8 in Do_Function ../src/core/c-function.c:415
    #24 0x420b7d in Apply_Function ../src/core/c-do.c:1528
    #25 0x42139c in Do_Sys_Func ../src/core/c-do.c:1588
    #26 0x426f84 in Init_Mezz ../src/core/c-do.c:2320
    #27 0x406bf1 in RL_Start ../src/core/a-lib.c:193
    #28 0x5fecee in main ../src/os/host-main.c:235
    #29 0x7facd0bf67ff in __libc_start_main (/usr/lib/libc.so.6+0x207ff)
    #30 0x406008 in _start (/home/zsx/stuffs/work/r3.git/make/r3-view-linux+0x406008)

0x60d00000c8a8 is located 0 bytes to the right of 136-byte region [0x60d00000c820,0x60d00000c8a8)
allocated by thread T0 here:
    #0 0x7facd26567a7 in malloc (/usr/lib/libasan.so.1+0x577a7)
    #1 0x4999a7 in Make_Mem ../src/core/m-pools.c:125
    #2 0x49b188 in Make_Series ../src/core/m-pools.c:413
    #3 0x521929 in Insert_Gobs ../src/core/t-gob.c:208
    #4 0x529717 in T_Gob ../src/core/t-gob.c:835
    #5 0x43eb86 in Do_Act ../src/core/c-function.c:338
    #6 0x43f82a in Do_Action ../src/core/c-function.c:396
    #7 0x419562 in Do_Next ../src/core/c-do.c:886
    #8 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #9 0x4a6eca in N_do ../src/core/n-control.c:524
    #10 0x43e267 in Do_Native ../src/core/c-function.c:289
    #11 0x419562 in Do_Next ../src/core/c-do.c:886
    #12 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #13 0x4a755e in N_either ../src/core/n-control.c:598
    #14 0x43e267 in Do_Native ../src/core/c-function.c:289
    #15 0x419562 in Do_Next ../src/core/c-do.c:886
    #16 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #17 0x4a755e in N_either ../src/core/n-control.c:598
    #18 0x43e267 in Do_Native ../src/core/c-function.c:289
    #19 0x419562 in Do_Next ../src/core/c-do.c:886
    #20 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #21 0x4a755e in N_either ../src/core/n-control.c:598
    #22 0x43e267 in Do_Native ../src/core/c-function.c:289
    #23 0x419562 in Do_Next ../src/core/c-do.c:886
    #24 0x41ad1d in Do_Blk ../src/core/c-do.c:1017
    #25 0x43f8f8 in Do_Function ../src/core/c-function.c:415
    #26 0x420b7d in Apply_Function ../src/core/c-do.c:1528
    #27 0x42139c in Do_Sys_Func ../src/core/c-do.c:1588
    #28 0x426f84 in Init_Mezz ../src/core/c-do.c:2320
    #29 0x406bf1 in RL_Start ../src/core/a-lib.c:193

SUMMARY: AddressSanitizer: heap-buffer-overflow ../src/core/t-gob.c:230 Insert_Gobs
Shadow bytes around the buggy address:
  0x0c1a7fff98c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff98d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff98e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff98f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1a7fff9900: fa fa fa fa 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c1a7fff9910: 00 00 00 00 00[fa]fa fa fa fa fa fa fa fa 00 00
  0x0c1a7fff9920: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa
  0x0c1a7fff9930: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x0c1a7fff9940: 00 00 00 00 00 00 00 00 00 fa fa fa fa fa fa fa
  0x0c1a7fff9950: fa fa 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c1a7fff9960: 00 00 00 00 fa fa fa fa fa fa fa fa fd fd fd fd
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Contiguous container OOB:fc
  ASan internal:           fe
==24248==ABORTING

src/core/t-gob.c

@@ -195,7 +195,7 @@ const REBCNT Gob_Flag_Words[] = {
 					}
 				}
 				Detach_Gob(VAL_GOB(val));
-				if ((REBINT)index > i) index--;
+				if (i >= 0 && (REBINT)index > i) index--;
 			}
 		} else {
 			Trap_Arg(val);