fix mallocng regression in malloc_usable_size with null argument

commit d1507646975cbf6c3e511ba07b193f27f032d108 added support for null
argument in oldmalloc and was overlooked when switching to mallocng.

src/malloc/mallocng/malloc_usable_size.c

@@ -3,6 +3,7 @@
 
 size_t malloc_usable_size(void *p)
 {
+	if (!p) return 0;
 	struct meta *g = get_meta(p);
 	int idx = get_slot_index(p);
 	size_t stride = get_stride(g);