[SCSI] aic79xx: check for non-NULL scb in ahd_handle_nonpkt_busfree

When removing several devices aic79xx will occasionally Oops
in ahd_handle_nonpkt_busfree during rescan. Looking at the
code I found that we're indeed not checking if the scb in
question is NULL. So check for it before accessing it.
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/aic7xxx/aic79xx_core.c

@@ -3171,13 +3171,16 @@ ahd_handle_nonpkt_busfree(struct ahd_softc *ahd)
 				tinfo->curr.transport_version = 2;
 				tinfo->goal.transport_version = 2;
 				tinfo->goal.ppr_options = 0;
+				if (scb != NULL) {
 					/*
-				 * Remove any SCBs in the waiting for selection
-				 * queue that may also be for this target so
-				 * that command ordering is preserved.
+					 * Remove any SCBs in the waiting
+					 * for selection queue that may
+					 * also be for this target so that
+					 * command ordering is preserved.
 					 */
 					ahd_freeze_devq(ahd, scb);
 					ahd_qinfifo_requeue_tail(ahd, scb);
+				}
 				printerror = 0;
 			}
 		} else if (ahd_sent_msg(ahd, AHDMSG_EXT, MSG_EXT_WDTR, FALSE)
@@ -3194,13 +3197,16 @@ ahd_handle_nonpkt_busfree(struct ahd_softc *ahd)
 				      MSG_EXT_WDTR_BUS_8_BIT,
 				      AHD_TRANS_CUR|AHD_TRANS_GOAL,
 				      /*paused*/TRUE);
+			if (scb != NULL) {
 				/*
-			 * Remove any SCBs in the waiting for selection
-			 * queue that may also be for this target so that
-			 * command ordering is preserved.
+				 * Remove any SCBs in the waiting for
+				 * selection queue that may also be for
+				 * this target so that command ordering
+				 * is preserved.
 				 */
 				ahd_freeze_devq(ahd, scb);
 				ahd_qinfifo_requeue_tail(ahd, scb);
+			}
 			printerror = 0;
 		} else if (ahd_sent_msg(ahd, AHDMSG_EXT, MSG_EXT_SDTR, FALSE)
 			&& ppr_busfree == 0) {
@@ -3217,13 +3223,16 @@ ahd_handle_nonpkt_busfree(struct ahd_softc *ahd)
 					/*ppr_options*/0,
 					AHD_TRANS_CUR|AHD_TRANS_GOAL,
 					/*paused*/TRUE);
+			if (scb != NULL) {
 				/*
-			 * Remove any SCBs in the waiting for selection
-			 * queue that may also be for this target so that
-			 * command ordering is preserved.
+				 * Remove any SCBs in the waiting for
+				 * selection queue that may also be for
+				 * this target so that command ordering
+				 * is preserved.
 				 */
 				ahd_freeze_devq(ahd, scb);
 				ahd_qinfifo_requeue_tail(ahd, scb);
+			}
 			printerror = 0;
 		} else if ((ahd->msg_flags & MSG_FLAG_EXPECT_IDE_BUSFREE) != 0
 			&& ahd_sent_msg(ahd, AHDMSG_1B,
@@ -3251,7 +3260,7 @@ ahd_handle_nonpkt_busfree(struct ahd_softc *ahd)
 	 * the message phases.  We check it last in case we
 	 * had to send some other message that caused a busfree.
 	 */
-	if (printerror != 0
+	if (scb != NULL && printerror != 0
 	 && (lastphase == P_MESGIN || lastphase == P_MESGOUT)
 	 && ((ahd->msg_flags & MSG_FLAG_EXPECT_PPR_BUSFREE) != 0)) {