Seg fault 'zpool import -d /dev/disk/by-id -a'

Introduced by commit 44867b6d6effc1628dd00c36821ab044f89fb988.
We should of course check to ensure best isn't NULL before
attempting to dereference it.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #974