Fix memory leaks in test_double_serializer, and make sure all tests return 0 at the end of main().

tests/test_parse.c

@@ -26,6 +26,8 @@ int main(void)
 	puts(separator);
 	test_incremental_parse();
 	puts(separator);
+
+	return 0;
 }
 
 static json_c_visit_userfunc clear_serializer;