alloc-util: don't use malloc_usable_size() to determine allocated size

This reverts commit d4b604baea.

When realloc() is called, the extra memory between the originally
requested size and the end of malloc_usable_size() isn't copied. (at
least with the version of glibc that currently ships on Arch Linux)
As a result, some elements get lost and use uninitialized memory, most
commonly 0, and can lead to crashes.

fixes #12384

src/basic/alloc-util.c

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
-#include <malloc.h>
 #include <stdint.h>
 #include <string.h>
 
@@ -65,7 +64,7 @@ void* greedy_realloc(void **p, size_t *allocated, size_t need, size_t size) {
                 return NULL;
 
         *p = q;
-        *allocated = _unlikely_(size == 0) ? newalloc : malloc_usable_size(q) / size;
+        *allocated = newalloc;
         return q;
 }