avformat/avidec: Simplify compile-time check for DV demuxer

1b373b41d9 made it a bit harder to find
out that a call to avpriv_dv_produce_packet is dead when the DV demuxer
is disabled; too hard for GCC on -O0. So simplify the check a bit.

Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/avidec.c

@@ -1483,7 +1483,7 @@ resync:
             }
         }
 
-        if (dv_demux) {
+        if (CONFIG_DV_DEMUXER && dv_demux) {
             AVBufferRef *avbuf = pkt->buf;
             size = avpriv_dv_produce_packet(avi->dv_demux, pkt,
                                             pkt->data, pkt->size, pkt->pos);