onscreen/native: Avoid callbacks on "detached" onscreens

Detached onscreens have no valid view so avoid servicing callbacks on
them during/after sleep mode. As previously mentioned in 45bda2d969.

Closes: https://launchpad.net/bugs/2020049

src/backends/native/meta-onscreen-native.c

@@ -1525,6 +1525,7 @@ try_post_latest_swap (CoglOnscreen *onscreen)
   MetaFrameNative *frame_native;
 
   if (onscreen_native->next_post.frame == NULL ||
+      onscreen_native->view == NULL ||
       meta_kms_is_shutting_down (kms))
     return;