Limit isoLevel to 1 and 2 to avoid segfaulting when isoLevel is set to 3 by
dereferencing a NULL function pointer

Add some asserts to ensure that isolevel is always either 1 or 2.

PR: 203645
Reported by: Thomas Schmitt <scdbackup@gmx.net>
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division