MFV r289530: 5847 libzfs_diff should check zfs_prop_get() return

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Albert Lee <trisk@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Alexander Eremin <a.eremin@nexenta.com>

illumos/illumos-gate@8430278980a48338e04c7dd52b495b7f1551367a