Corrected a case where we could read uninited ABD memory

For my sins, I started running valgrind over ztest to try and fix
that pesky intermittent "zloop dies with malloc errors" problem.

This one seemed exciting enough to merit cutting a PR for before
the rest get polished.

Suggested-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12214