Use after free in r_core_magic_at

[hannob]

This script will cause a use after free error:
https://crashes.fuzzing-project.org/radare2-script-uaf-heap-r_core_magic_at

It contains the command "pm0" twice. Found with american fuzzy lop.
Test: radare2 -q -i [script] /dev/null

==4421==ERROR: AddressSanitizer: heap-use-after-free on address 0x6020000168b2 at pc 0x7fbc9a9c25b5 bp 0x7ffff981f960 sp 0x7ffff981f920
READ of size 1 at 0x6020000168b2 thread T0
    #0 0x7fbc9a9c25b4 in strcmp (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x385b4)
    #1 0x7fbc9a2d5ec5 in r_core_magic_at /f/radare2/radare2/libr/core/cmd_magic.c:41
    #2 0x7fbc9a2e9740 in r_core_magic /f/radare2/radare2/libr/core/cmd_magic.c:155
    #3 0x7fbc9a2e9740 in cmd_print /f/radare2/radare2/libr/core/cmd_print.c:2338
    #4 0x7fbc9a35323d in r_core_cmd_subst_i /f/radare2/radare2/libr/core/cmd.c:1590
    #5 0x7fbc9a2b702c in r_core_cmd_subst /f/radare2/radare2/libr/core/cmd.c:1081
    #6 0x7fbc9a2b82ee in r_core_cmd /f/radare2/radare2/libr/core/cmd.c:1938
    #7 0x7fbc9a2bb11c in r_core_cmd_lines /f/radare2/radare2/libr/core/cmd.c:1989
    #8 0x7fbc9a2bb38e in r_core_cmd_file /f/radare2/radare2/libr/core/cmd.c:2001
    #9 0x7fbc9a2be363 in r_core_run_script /f/radare2/radare2/libr/core/cmd.c:373
    #10 0x405572 in main /f/radare2/radare2/binr/radare2/radare2.c:729
    #11 0x7fbc9481bf9f in __libc_start_main (/lib64/libc.so.6+0x1ff9f)
    #12 0x40a333 (/mnt/ram/radare2/radare2+0x40a333)

0x6020000168b2 is located 2 bytes inside of 4-byte region [0x6020000168b0,0x6020000168b4)
freed by thread T0 here:
    #0 0x7fbc9a9e147f in __interceptor_free (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x5747f)
    #1 0x7fbc9a2b6dad in r_core_cmd_subst /f/radare2/radare2/libr/core/cmd.c:1105

previously allocated by thread T0 here:
    #0 0x7fbc9a9e16f7 in malloc (/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/libasan.so.1+0x576f7)
    #1 0x7fbc9487d789 in strdup (/lib64/libc.so.6+0x81789)

SUMMARY: AddressSanitizer: heap-use-after-free ??:0 strcmp
Shadow bytes around the buggy address:
  0x0c047fffacc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffacd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047ffface0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fffacf0: fa fa fa fa fa fa fa fa fa fa 01 fa fa fa 04 fa
  0x0c047fffad00: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
=>0x0c047fffad10: fa fa fd fa fa fa[fd]fa fa fa 00 fa fa fa 00 fa
  0x0c047fffad20: fa fa fd fa fa fa fd fd fa fa 06 fa fa fa fd fa
  0x0c047fffad30: fa fa 05 fa fa fa fd fa fa fa 06 fa fa fa fd fa
  0x0c047fffad40: fa fa 03 fa fa fa fd fa fa fa 04 fa fa fa fd fa
  0x0c047fffad50: fa fa fd fa fa fa 04 fa fa fa fd fa fa fa fd fa
  0x0c047fffad60: fa fa fd fa fa fa fd fa fa fa 01 fa fa fa fd fa
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
==4421==ABORTING