target: Fix free-after-use regression in PR unregister

This patch addresses a >= v3.11 free-after-use regression
in core_scsi3_emulate_pro_register() that was introduced
in the following commit:

commit bc118fe4
Author: Andy Grover <agrover@redhat.com>
Date:   Thu May 16 10:41:04 2013 -0700

    target: Further refactoring of core_scsi3_emulate_pro_register()

To avoid the free-after-use, save an type value before hand, and
only call core_scsi3_put_pr_reg() with a valid *pr_reg.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: <stable@vger.kernel.org> #3.11+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>