Fix crash caused by invalid snapshot names in redactnvl

This is a follow up fix for commit 0fdd6106bb. The VERIFY is
only true when we haven't hit an error code path. See added
test case for a reproducer.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11048