fix oops in scsi_run_queue()

fix oops in scsi_run_queue()

commit c055f5b2614b4f758ae6cc86733f31fa4c2c5844 upstream.

The recent commit closing the race window in device teardown:

commit 86cbfb5607d4b81b1a993ff689bbd2addd5d3a9b
Author: James Bottomley <James.Bottomley@suse.de>
Date:   Fri Apr 22 10:39:59 2011 -0500

    [SCSI] put stricter guards on queue dead checks

is causing a potential NULL deref in scsi_run_queue() because the
q->queuedata may already be NULL by the time this function is called.
Since we shouldn't be running a queue that is being torn down, simply
add a NULL check in scsi_run_queue() to forestall this.

Tested-by: Jim Schutt <jaschut@sandia.gov>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>