grabHelper: Ensure we reset ignoreRelease when we drop our event handler

Since we drop our event handler, we won't see ever the release event
from the button press, so unset the flag so it doesn't muck with the
next time somebody takes a grab.

https://bugzilla.gnome.org/show_bug.cgi?id=696102

js/ui/grabHelper.js

@@ -308,6 +308,8 @@ const GrabHelper = new Lang.Class({
         if (!this.grabbed && this._capturedEventId > 0) {
             global.stage.disconnect(this._capturedEventId);
             this._capturedEventId = 0;
+
+            this._ignoreRelease = false;
         }
 
         if (hadFocus) {