workspacesDisplay: Cancel click when panning

When switching between workspaces via panning, we don't want to
leave the overview when we end up on an empty workspace.

https://bugzilla.gnome.org/show_bug.cgi?id=766883

js/ui/workspacesView.js

@@ -436,6 +436,7 @@ const WorkspacesDisplay = new Lang.Class({
             return true;
         }));
         panAction.connect('gesture-cancel', Lang.bind(this, function() {
+            clickAction.release();
             for (let i = 0; i < this._workspacesViews.length; i++)
                 this._workspacesViews[i].endSwipeScroll();
         }));