Poison and unpoison on realloc

Prior to this change, the asan guard size meant that the fast path for realloc never triggered in asan mode, masking the fact that the realloc fast path did not unpoison the newly extended memory region. This change also poisons the "previous" memory area if an alloc+copy is performed, to match the C realloc behavior of freeing the previous memory.

PiperOrigin-RevId: 734278556