block/sheepdog: remove spurious NULL check

'tag' is already checked in the lines immediately preceding this check,
and set to non-NULL if NULL.  No need to check again, it hasn't changed.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
