grabHelper: Correct typo preventing focus-window-changed disconnect

While debugging, I found that the signal to focus-window-changed
was never getting disconnected, making a call to ungrab every time
the focus window changed, even if there were no focus grabs anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=693975

js/ui/grabHelper.js

@@ -232,7 +232,7 @@ const GrabHelper = new Lang.Class({
             this._keyFocusNotifyId = 0;
         }
 
-        if (!this._focusWindowChanged > 0) {
+        if (this._focusWindowChangedId > 0) {
             let metaDisplay = global.screen.get_display();
             metaDisplay.disconnect(this._focusWindowChangedId);
             this._focusWindowChangedId = 0;