dnd: don't try to access destroyed dragActor

The dragComplete handler incorrectly checks
this._actorDestroyed to see if the drag actor
is destroyed.  The drag actor may not be the same
as the main actor.

The end result is an exception in drop handling,
leading to a shell lockup.

This commit changes the code to always set
this._dragActor to undefined when it's destroyed,
and check for that condition instead of
this._actorDestroyed in the dragComplete handler.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/issues/577

js/ui/dnd.js

@@ -506,6 +506,7 @@ var _Draggable = new Lang.Class({
                             return true;
                         } else
                             this._dragActor.destroy();
+                            this._dragActor = undefined;
                     }
 
                     this._dragInProgress = false;
@@ -565,8 +566,10 @@ var _Draggable = new Lang.Class({
             if (!this._buttonDown)
                 this._dragComplete();
             this.emit('drag-end', eventTime, false);
-            if (!this._dragOrigParent)
+            if (!this._dragOrigParent) {
                 this._dragActor.destroy();
+                this._dragActor = undefined;
+            }
 
             return;
         }
@@ -634,6 +637,7 @@ var _Draggable = new Lang.Class({
             dragActor.set_position(this._dragOrigX, this._dragOrigY);
         } else {
             dragActor.destroy();
+            this._dragActor = undefined;
         }
 
         this.emit('drag-end', eventTime, false);
@@ -641,7 +645,7 @@ var _Draggable = new Lang.Class({
     },
 
     _dragComplete() {
-        if (!this._actorDestroyed)
+        if (this._dragActor)
             Shell.util_set_hidden_from_pick(this._dragActor, false);
 
         this._ungrabEvents();