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 [];
         },