ABD: Be more assertive in iterators

Once we verified the ABDs and asserted the sizes we should never
see premature ABDs ends. Assert that and remove extra branches
from production builds.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15428