MFV 286542: 5592 NULL pointer dereference in dsl_prop_notify_all_cb()

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>

illumos/illumos-gate@9d47dec0481d8cd53b2c1053c96bfa3f78357d6a