Illumos 5592 - NULL pointer dereference in dsl_prop_notify_all_cb()

5592 NULL pointer dereference in dsl_prop_notify_all_cb()
Author: Justin T. Gibbs <justing@spectralogic.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Approved by: Robert Mustacchi <rm@joyent.com>

References:

https://www.illumos.org/issues/5592
https://github.com/illumos/illumos-gate/commit/9d47dec

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