rmdec: don't set codec timebase.

It's not supposed to be set outside of lavc.
Set r_frame_rate instead.

libavformat/rmdec.c

@@ -332,10 +332,9 @@ ff_rm_read_mdpr_codecdata (AVFormatContext *s, AVIOContext *pb,
         if ((ret = rm_read_extradata(pb, st->codec, codec_data_size - (avio_tell(pb) - codec_pos))) < 0)
             return ret;
 
-        av_reduce(&st->codec->time_base.num, &st->codec->time_base.den,
+        av_reduce(&st->r_frame_rate.den, &st->r_frame_rate.num,
                   0x10000, fps, (1 << 30) - 1);
-        st->avg_frame_rate.num = st->codec->time_base.den;
-        st->avg_frame_rate.den = st->codec->time_base.num;
+        st->avg_frame_rate = st->r_frame_rate;
     }
 
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