json: Make it obvious that ast_json_unref() is NULL safe.

It looked like the safety check was done after the NULL pointer was used.
........

Merged revisions 399576 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399578 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/json.c

@@ -213,13 +213,15 @@ void ast_json_unref(struct ast_json *json)
 	struct json_mem_list *free_list;
 	struct json_mem *mem;
 
+	if (!json) {
+		return;
+	}
+
 	/* Jansson refcounting is non-atomic; lock it. */
 	{
 		SCOPED_JSON_LOCK(json);
-		if (!json) {
-			return;
-		}
-		json_decref((json_t *)json);
+
+		json_decref((json_t *) json);
 	}
 
 	/* Now free any objects that were ast_json_free()'s while the lock was