Illumos #4089 NULL pointer dereference in arc_read()

4089 NULL pointer dereference in arc_read()

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:
  https://www.illumos.org/issues/4089
  illumos/illumos-gate@57815f6b95a743697e148327725b7f568e75e6ea

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2171