Reset surface transientParent when the parent is destroyed.

If a transient parent of a surface is destroyed before the surface
itself, reset all references to that parent surface.

Change-Id: I5b748099d8a5502632d8846c5de5381b5f28b665
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>

src/compositor/wayland_wrapper/qwlsurface.cpp

@@ -408,6 +408,14 @@ void Surface::surface_destroy_resource(Resource *)
         m_extendedSurface = 0;
     }
 
+    if (transientParent()) {
+        foreach (Surface *surface, compositor()->surfaces()) {
+            if (surface->transientParent() == this) {
+                surface->setTransientParent(0);
+            }
+        }
+    }
+
     m_destroyed = true;
     m_waylandSurface->destroy();
     emit m_waylandSurface->surfaceDestroyed();