avformat/demux: Count EAGAIN as 100 bytes in relation to read limit in avformat_find_stream_info()

Fixes: Timeout
Fixes: 43717/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-5206008287330304
Fixes: 45738/clusterfuzz-testcase-minimized-ffmpeg_IO_DEMUXER_fuzzer-6142535657979904

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

libavformat/demux.c

@@ -2615,8 +2615,10 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
         /* NOTE: A new stream can be added there if no header in file
          * (AVFMTCTX_NOHEADER). */
         ret = read_frame_internal(ic, pkt1);
-        if (ret == AVERROR(EAGAIN))
+        if (ret == AVERROR(EAGAIN)) {
+            read_size += 100;
             continue;
+        }
 
         if (ret < 0) {
             /* EOF or error*/