alloc-util: introduce MALLOC_SIZEOF_SAFE() helper

It's a wrapper around malloc_usable_size() that is supposed to be
compatible with _FORTIFY_SOURCES=1, by taking the
__builtin_object_size() data into account, the same way as the
_FORTIFY_SOURCES=1 logic does.

Fixes: #19203

src/basic/memory-util.h

@@ -88,7 +88,7 @@ static inline void* erase_and_free(void *p) {
         if (!p)
                 return NULL;
 
-        l = malloc_usable_size(p);
+        l = MALLOC_SIZEOF_SAFE(p);
         explicit_bzero_safe(p, l);
         return mfree(p);
 }