librbd/object_map: don't assert if a snapshot doesn't exist

librbd/object_map: don't assert if a snapshot doesn't exist

When compute a fast-diff, if an snapshot no longer exists or fails
to load, ignore the error as long as it wasn't the start or end
snapshot.

Fixes: https://tracker.ceph.com/issues/48526
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit abab5f143b80ff876948307f00ab767bc1b6ba22)

Conflicts:
	src/test/librbd/object_map/test_mock_DiffRequest.cc: exec() mock only takes 7 params