self referencing variable gives a proper error (no stack overflow). Fixes #972

lib/less/tree/variable.js

@@ -8,12 +8,24 @@ tree.Variable.prototype = {
         if (name.indexOf('@@') == 0) {
             name = '@' + new(tree.Variable)(name.slice(1)).eval(env).value;
         }
+        
+        if (this.evaluating) {
+            throw { type: 'Name',
+                    message: "Recursive variable definition for " + name,
+                    filename: this.file,
+                    index: this.index };
+        }
+        
+        this.evaluating = true;
 
         if (variable = tree.find(env.frames, function (frame) {
             if (v = frame.variable(name)) {
                 return v.value.eval(env);
             }
-        })) { return variable }
+        })) { 
+            this.evaluating = false;
+            return variable;
+        }
         else {
             throw { type: 'Name',
                     message: "variable " + name + " is undefined",

test/less/errors/recursive-variable.less

@@ -0,0 +1 @@
+@bodyColor: darken(@bodyColor, 30%);

test/less/errors/recursive-variable.txt

@@ -0,0 +1,2 @@
+NameError: Recursive variable definition for @bodyColor in {path}recursive-variable.less:1:19
+1 @bodyColor: darken(@bodyColor, 30%);