aic79xx: check for non-NULL scb in ahd_handle_nonpkt_busfree

aic79xx: check for non-NULL scb in ahd_handle_nonpkt_busfree

commit 534ef056db8a8fb6b9d50188d88ed5d1fbc66673 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>