dm snapshot: revert "fix a crash when an origin has no snapshots"

Commit 7ee06ddc ("dm snapshot: fix a
crash when an origin has no snapshots") introduced a regression in
snapshot merging - causing the lvm2 test lvcreate-cache-snapshot.sh
got stuck in an infinite loop.

Even though commit 7ee06ddc was marked
for stable@ the stable team was notified to _not_ backport it.

Fixes: 7ee06ddc ("dm snapshot: fix a crash when an origin has no snapshots")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>