Illumos 5531 - NULL pointer dereference in dsl_prop_get_ds()

5531 NULL pointer dereference in dsl_prop_get_ds()
Author: Justin T. Gibbs <justing@spectralogic.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Bayard Bell <buffer.g.overflow@gmail.com>
Approved by: Robert Mustacchi <rm@joyent.com>

References:

https://www.illumos.org/issues/5531
https://github.com/illumos/illumos-gate/commit/e57a022

Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>