UnlockDialog: clear the password on failure

It is wrong, and the user can't correct it because it's obfuscated.
Just let him type it again.

https://bugzilla.gnome.org/show_bug.cgi?id=687132

js/ui/unlockDialog.js

@@ -283,6 +283,7 @@ const UnlockDialog = new Lang.Class({
         this._currentQuery = null;
         this._firstQuestion = true;
 
+        this._promptEntry.text = '';
         this._promptEntry.clutter_text.set_password_char('\u25cf');
         this._promptEntry.menu.isPassword = true;