Prevent context overwrite by replaceStack

Fixes #408.

lib/handlebars/compiler/compiler.js

@@ -886,21 +886,28 @@ Handlebars.JavaScriptCompiler = function() {};
     },
 
     pushStack: function(item) {
-      this.source.push(this.incrStack() + " = " + item + ";");
-      this.compileStack.push("stack" + this.stackSlot);
-      return "stack" + this.stackSlot;
+      var stack = this.incrStack();
+      this.source.push(stack + " = " + item + ";");
+      this.compileStack.push(stack);
+      return stack;
     },
 
     replaceStack: function(callback) {
-      var item = callback.call(this, this.topStack());
+      var stack = this.topStack(),
+          item = callback.call(this, stack);
 
-      this.source.push(this.topStack() + " = " + item + ";");
-      return "stack" + this.stackSlot;
+      // Prevent modification of the context depth variable. Through replaceStack
+      if (this.compileStack.length <= 1) {
+        stack = this.nextStack();
+      }
+
+      this.source.push(stack + " = " + item + ";");
+      return stack;
     },
 
     nextStack: function(skipCompileStack) {
       var name = this.incrStack();
-      this.compileStack.push("stack" + this.stackSlot);
+      this.compileStack.push(name);
       return name;
     },