When modifying a RAID stripe-extent, ASSERT() that the length of the new
RAID stripe-extent is always greater than 0.
Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: David Sterba <dsterba@suse.com>