dnd: Don't try to remove an invalid idle source

As the handler returns false, the corresponding source is removed
automatically and its id invalidated. Reset the id to 0 to reflect
this, otherwise newer versions of GLib will print a warning when
we later try to remove it explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=711555

js/ui/dnd.js

@@ -346,6 +346,7 @@ const _Draggable = new Lang.Class({
     },
 
     _updateDragHover : function () {
+        this._updateHoverId = 0;
         let target = this._dragActor.get_stage().get_actor_at_pos(Clutter.PickMode.ALL,
                                                                   this._dragX, this._dragY);
         let dragEvent = {