[iscsi] Report SCSI response only when applicable

iSCSI generally includes a full SCSI response only when an error
occurs.  iscsi_scsi_done() currently passes the NULL response through
to scsi_response(), which ends up causing scsicmd_response() to
dereference a NULL pointer.

Fix by calling scsi_response() only if we have a non-NULL response.

Reported-by: Brendon Walsh <brendonwalsh@niamu.com>
Tested-by: Brendon Walsh <brendonwalsh@niamu.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/iscsi.c

 	iscsi->command = NULL;
 
 	/* Send SCSI response, if any */
-	scsi_response ( &iscsi->data, rsp );
+	if ( rsp )
+		scsi_response ( &iscsi->data, rsp );
 
 	/* Close SCSI command, if this is still the same command.  (It
 	 * is possible that the command interface has already been