Improve robustness of the code that restores .queue_pdu()

In my tests I hit a stack overflow caused by infinite recursion of
.queue_pdu() calls, caused by .queue_pdu() not being restored in an
error path. Make the approach for restoring .queue_pdu() more robust.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

test-tool/iscsi-test-cu.h

@@ -35,6 +35,8 @@
 /* globals between setup, tests, and teardown */
 extern struct scsi_task *task;
 extern unsigned char *read_write_buf;
+extern int (*orig_queue_pdu)(struct iscsi_context *iscsi,
+                             struct iscsi_pdu *pdu);
 
 #ifndef HAVE_CU_SUITEINFO_PSETUPFUNC
 /* libcunit version 1 */