When the backup-top node transitions from active to inactive in
bdrv_backup_top_drop(), the BlockCopyState is freed and the filtered
child is removed, so the node effectively becomes unusable.

However, noone told its I/O functions this, so they will happily
continue accessing bs->backing and s->bcs.  Prevent that by aborting
early when s->active is false.

(After the preceding patch, the node should be gone after
bdrv_backup_top_drop(), so this should largely be a theoretical problem.
But still, better to be safe than sorry, and also I think it just makes
sense to check s->active in the I/O functions.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210219153348.41861-3-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>