Fix size of register buffer

When calling a function with double arguments, I get this asan error:

==7920==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x0053131ece38 at pc 0x7ff79697a68f bp 0x0053131ec790 sp 0x0053131ebf40
READ of size 16 at 0x0053131ece38 thread T0
    #0 0x7ff79697a68e in MemcmpInterceptorCommon(void*, int (*)(void const*, void const*, unsigned long long), void const*, void const*, unsigned long long) C:/gcc/src/gcc-14.2.0/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:814
    #1 0x7ff79697aebd in memcmp C:/gcc/src/gcc-14.2.0/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:845
    #2 0x7ff79697aebd in memcmp C:/gcc/src/gcc-14.2.0/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:840
    #3 0x7ff7927e237f in regcache::raw_write(int, gdb::array_view<unsigned char const>) C:/gdb/src/gdb.git/gdb/regcache.c:874
    #4 0x7ff7927e3c85 in regcache::cooked_write(int, gdb::array_view<unsigned char const>) C:/gdb/src/gdb.git/gdb/regcache.c:914
    #5 0x7ff7927e5d89 in regcache::cooked_write(int, unsigned char const*) C:/gdb/src/gdb.git/gdb/regcache.c:933
    #6 0x7ff7911d5965 in amd64_windows_store_arg_in_reg C:/gdb/src/gdb.git/gdb/amd64-windows-tdep.c:216

Address 0x0053131ece38 is located in stack of thread T0 at offset 40 in frame
    #0 0x7ff7911d565f in amd64_windows_store_arg_in_reg C:/gdb/src/gdb.git/gdb/amd64-windows-tdep.c:208

  This frame has 4 object(s):
    [32, 40) 'buf' (line 211) <== Memory access at offset 40 overflows this variable

It's because the first 4 double arguments are passed via XMM registers,
and they need a buffer of 16 bytes, even if we only use 8 bytes of them.

Approved-By: Tom Tromey <[email protected]>

gdb/amd64-windows-tdep.c

@@ -207,7 +207,8 @@ amd64_windows_store_arg_in_reg (struct regcache *regcache,
 {
   struct type *type = arg->type ();
   const gdb_byte *valbuf = arg->contents ().data ();
-  gdb_byte buf[8];
+  /* We only set 8 bytes, buf if it's a XMM register, 16 bytes are read.  */
+  gdb_byte buf[16];
 
   gdb_assert (type->length () <= 8);
   memset (buf, 0, sizeof buf);