scsi: remove superfluous NULL pointer check from scsi_kill_request()

Dan's list included:

drivers/scsi/scsi_lib.c +1365 scsi_kill_request(9) warning: variable derefenced in initializer 'cmd'
drivers/scsi/scsi_lib.c +1365 scsi_kill_request(9) warning: variable derefenced before check 'cmd'

We dereference cmd (and possible OOPS if cmd == NULL) before starting the
request so just remove the superfluous debugging code altogether.

[ bart: the potential NULL pointer dereference was finally fixed in
  (much later than mine) commit 03b14708 but my patch is still valid ]
Reported-by: Dan Carpenter <error27@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Eugene Teo <eteo@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/scsi/scsi_lib.c

@@ -1370,12 +1370,6 @@ static void scsi_kill_request(struct request *req, struct request_queue *q)
 
 	blk_start_request(req);
 
-	if (unlikely(cmd == NULL)) {
-		printk(KERN_CRIT "impossible request in %s.\n",
-				 __func__);
-		BUG();
-	}
-
 	sdev = cmd->device;
 	starget = scsi_target(sdev);
 	shost = sdev->host;