Fix double toCSS calls in unusual situations causing an exception. Fixes #1830

lib/less/to-css-visitor.js

@@ -18,6 +18,9 @@
         },
 
         visitMixinDefinition: function (mixinNode, visitArgs) {
+            // mixin definitions do not get eval'd - this means they keep state
+            // so we have to clear that state here so it isn't used if toCSS is called twice
+            mixinNode.frames = [];
             return [];
         },
 

test/less-test.js

@@ -227,6 +227,8 @@ module.exports = function() {
                 } else {
                     try {
                         css = tree.toCSS(options);
+                        var css2 = tree.toCSS(options); // integration test that 2nd call gets same output
+                        if (css2 !== css) { throw new Error("css not equal to 2nd call"); }
                         callback(null, css);
                     } catch (e) {
                         callback(e);