avformat/mxfenc: dont warn about d10_channelcount being ignored if its not set

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfenc.c

@@ -1023,7 +1023,7 @@ static void mxf_write_generic_sound_common(AVFormatContext *s, AVStream *st, con
             av_log(s, AV_LOG_WARNING, "d10_channelcount shall be set to 4 or 8 : the output will not comply to MXF D-10 specs\n");
         avio_wb32(pb, mxf->channel_count);
     } else {
-        if (show_warnings)
+        if (show_warnings && mxf->channel_count != -1)
             av_log(s, AV_LOG_ERROR, "-d10_channelcount requires MXF D-10 and will be ignored\n");
         avio_wb32(pb, st->codec->channels);
     }