Stop segfaulting on unmount error case

After interrupting ZTS runs that errored out, I found that
"zpool export testpool2" was segfaulting.

This seems unnecessary.

Reviewed-by: szubersk <szuberskidamian@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12804