[XForms] Fix changed nodes error on form initialization. Bug 331469, r=doronr+smaug

extensions/xforms/nsXFormsModelElement.cpp

@@ -1546,6 +1546,8 @@ nsXFormsModelElement::InitializeControls()
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
+  mChangedNodes.Clear();
+
   return NS_OK;
 }