[ENOMEM][fuzzer/subset] early return if the result is null

I don't see _or_fail APIs idiomatic for the project but since it is there, let's have this

test/fuzzing/hb-subset-fuzzer.cc

@@ -16,6 +16,7 @@ trySubset (hb_face_t *face,
 	   bool retain_gids)
 {
   hb_subset_input_t *input = hb_subset_input_create_or_fail ();
+	if (!input) return;
   hb_subset_input_set_drop_hints (input, drop_hints);
   hb_subset_input_set_retain_gids (input, retain_gids);
   hb_set_t *codepoints = hb_subset_input_unicode_set (input);