Illumos 5847 - libzfs_diff should check zfs_prop_get() return

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>

References:

https://www.illumos.org/issues/5847
https://github.com/illumos/illumos-gate/commit/8430278

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3412