avformat/flvdec: Set need_context_update when setting the initial extradata

Fixes ticket 6398.

Debugged with the help of James Almer and Hendrik Leppkes.

libavformat/flvdec.c

@@ -754,6 +754,7 @@ static int flv_get_extradata(AVFormatContext *s, AVStream *st, int size)
     av_freep(&st->codecpar->extradata);
     if (ff_get_extradata(s, st->codecpar, s->pb, size) < 0)
         return AVERROR(ENOMEM);
+    st->internal->need_context_update = 1;
     return 0;
 }