Fix callback called two times

lib/less/parser.js

@@ -309,17 +309,21 @@ less.Parser = function Parser(env) {
                     }
                 }
                 if (level > 0) {
-                    return callback(new(LessError)({
+                    error = new(LessError)({
                         index: i,
                         type: 'Parse',
                         message: "missing closing `}`",
                         filename: env.filename
-                    }, env));
+                    }, env);
                 }
 
                 return chunks.map(function (c) { return c.join('') });;
             })([[]]);
 
+            if (error) {
+                return callback(error);
+            }
+
             // Start with the primary rule.
             // The whole syntax tree is held under a Ruleset node,
             // with the `root` property set to true, so no `{}` are