screenShield: fix empty screen shield

If the drag action ends after something else has put the screen shield
into a different state we can end up in an inconsistent screen shield
state where the whole thing is empty.

https://bugzilla.gnome.org/show_bug.cgi?id=703126

js/ui/screenShield.js

@@ -720,6 +720,8 @@ const ScreenShield = new Lang.Class({
     },
 
     _onDragEnd: function(action, actor, eventX, eventY, modifiers) {
+        if (this._lockScreenState != MessageTray.State.HIDING)
+            return;
         if (this._lockScreenGroup.y < -(ARROW_DRAG_THRESHOLD * global.stage.height)) {
             // Complete motion automatically
 	    let [velocity, velocityX, velocityY] = this._dragAction.get_velocity(0);