tests: Avoid calling malloc(0) in gvariant tests

Its behaviour is implementation-defined according to POSIX
(https://pubs.opengroup.org/onlinepubs/9699919799/functions/malloc.html),
and we’d quite like it to consistently return `NULL` for a zero-size
allocation.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/tests/gvariant.c

@@ -1352,7 +1352,7 @@ align_malloc (gsize size)
    * have malloc() that returns non-8-aligned.  if so, we need to try
    * harder here.
    */
-  mem = malloc (size);
+  mem = (size > 0) ? malloc (size) : NULL;
 #endif
 
   return mem;