UBSAN should catch undefined behavior in realloc

[bhaible]

ISO C 23 § 7.24.3.7 specifies that realloc (ptr, 0), when ptr is non-null, is "undefined behavior".

It would be very useful if the UBSAN would catch such invocations, because this corner of realloc's specification is a real portability hassle, cf. https://sourceware.org/bugzilla/show_bug.cgi?id=12547 .

How to reproduce:
========================== foo.c =========================

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
int main ()
{
  char *p = malloc (200);
  printf ("%p", p);
  errno = 0;
  char *q = realloc (p, 0);
  printf (" %p %d\n", q, errno);
}

$ clang -Wall -fsanitize=undefined -std=gnu23 foo.c
$ ASAN_OPTIONS="abort_on_error=0 allocator_may_return_null=1" UBSAN_OPTIONS= ./a.out 
0x55fd545a9300 (nil) 0
$ ASAN_OPTIONS="abort_on_error=0 allocator_may_return_null=0" UBSAN_OPTIONS= ./a.out 
0x5620c7431300 (nil) 0

Expected: Some error report from UBSAN.

Seen with clang version 19.

[jcking]

This is likely more appropriate for ASan and HWASan, which will be updated soon to handle free_sized and free_aligned_sized, and I will likely make them complain when somebody passes bad alignment or size.

[jcking]

Unless somebody has an objection, I'll move to make ASan and HWASan report this as an error that can be disabled via flags, while I am working on free_sized and free_aligned_sized.