avformat/avidec: Fix memleak when DV demuxer is disabled

If one uses a build without dv demuxer, an AVIStream struct that is
destined to be used as private data for an AVStream by the avi demuxer
would leak, because it has been moved from the AVStream (that is going
to be freed) and only stored in a local variable (in order to be used
for another AVStream), but if the dv demuxer is disabled, the earlier
code returned immediately instead.

Also return a better error code in this scenario (instead of
AVERROR_INVALIDDATA).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/avidec.c

@@ -599,15 +599,16 @@ static int avi_read_header(AVFormatContext *s)
                     handler != MKTAG('d', 'v', 's', 'l'))
                     goto fail;
 
+                if (!CONFIG_DV_DEMUXER)
+                    return AVERROR_DEMUXER_NOT_FOUND;
+
                 ast = s->streams[0]->priv_data;
                 st->priv_data = NULL;
                 ff_free_stream(s, st);
-                if (CONFIG_DV_DEMUXER) {
                     avi->dv_demux = avpriv_dv_init_demux(s);
                     if (!avi->dv_demux)
                         goto fail;
-                } else
-                    goto fail;
+
                 s->streams[0]->priv_data = ast;
                 avio_skip(pb, 3 * 4);
                 ast->scale = avio_rl32(pb);