block: allow bdrv_unref() to be passed NULL pointers If bdrv_unref() is passed a NULL BDS pointer, it is safe to exit with no operation. This will allow cleanup code to blindly call bdrv_unref() on a BDS that has been initialized to NULL. Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Jeff Cody <jcody@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>