Return NULL from `sub_alloc` for zero size

When zero was passed for the size to `sub_alloc`, we were passing this
size on to `malloc` or `calloc`, which is unusual enough that static
analyzers warn about this (POSIX says that either NULL or a pointer
will be returned).

We'll instead just return NULL right away.

libs/sofia-sip/libsofia-sip-ua/su/su_alloc.c

@@ -432,6 +432,8 @@ void *sub_alloc(su_home_t *home,
   if (size >= ((size_t)1) << SIZEBITS)
     return (void)(errno = ENOMEM), NULL;
 
+  if (!size) return NULL;
+
   if (sub == NULL || 3 * sub->sub_used > 2 * sub->sub_n) {
     /* Resize the hash table */
     size_t i, n, n2;
@@ -474,7 +476,7 @@ void *sub_alloc(su_home_t *home,
     sub = b2;
   }
 
-  if (size && sub && zero < do_clone &&
+  if (sub && zero < do_clone &&
       sub->sub_preload && size <= sub->sub_prsize) {
     /* Use preloaded memory */
     size_t prused = sub->sub_prused + size + MEMCHECK_EXTRA;