avformat/usmdec: do not return 0 when no packet was produced

Fixes: Assertion pkt->stream_index < (unsigned)s->nb_streams && "Invalid stream index.\n" failed at libavformat/demux.c:617
Fixes: 62498/clusterfuzz-testcase-minimized-ffmpeg_dem_USM_fuzzer-4734740995112960

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/usmdec.c

@@ -361,7 +361,7 @@ static int64_t parse_chunk(AVFormatContext *s, AVIOContext *pb,
     ret = avio_skip(pb, FFMAX(0, chunk_size - (ret - chunk_start)));
     if (ret < 0)
         return ret;
-    return 0;
+    return FFERROR_REDO;
 }
 
 static int usm_read_packet(AVFormatContext *s, AVPacket *pkt)