target/pscsi: Fix bug with REPORT_LUNs handling for SCSI passthrough

This patch fixes a regression bug in pscsi_transport_complete() callback
code where *pt was being NULL dereferenced during REPORT_LUNS handling,
that was introduced with the spc/sbc refactoring in:

commit 1fd032ee10d2816c947f5d5b9abda95e728f0a8f
Author: Christoph Hellwig <hch@infradead.org>
Date:   Sun May 20 11:59:15 2012 -0400

    target: move code for CDB emulation

As this is a special case for pscsi_parse_cdb() to call spc_parse_cdb() to
allow TCM to handle REPORT_LUN emulation, pscsi_plugin_task will have not
been allocated..

So now in pscsi_transport_complete() just check for existence of *pt and
return for this special case.

Reported-by: Alex Elsayed <eternaleye+usenet@gmail.com>
Cc: Alex Elsayed <eternaleye+usenet@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>