check groupName and uid on groups.join

src/groups/membership.js

@@ -44,6 +44,10 @@ module.exports = function(Groups) {
 
 		callback = callback || function() {};
 
+		if (!groupName || !parseInt(uid, 10)) {
+			return callback(new Error('[[error:invalid-data]]'));
+		}
+
 		Groups.exists(groupName, function(err, exists) {
 			if (err) {
 				return callback(err);