MFV r258377: 4088 use after free in arc_release()

illumos/illumos-gate@ccc22e130479b5bd7c0002267fee1e0602d3f772

MFC after:	5 days
MFV r258378: 4089 NULL pointer dereference in arc_read()

illumos/illumos-gate@57815f6b95a743697e148327725b7f568e75e6ea

Tested by:	adrian
MFC after:	4 days