V4L/DVB (13968): cx18, fix potential null dereference

Stanse found a potential null dereference in cx18_dvb_start_feed
and cx18_dvb_stop_feed. There is a check for stream being NULL,
but it is dereferenced earlier. Move the dereference after the
check.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Andy Walls <awalls@radix.net>
Acked-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>