avformat/asfdec_f: Check for negative ext_len

Fixes: Infinite loop
Fixes: 26376/clusterfuzz-testcase-minimized-ffmpeg_dem_PCM_U32LE_fuzzer-6050518830678016
Fixes: 26377/clusterfuzz-testcase-minimized-ffmpeg_dem_TY_fuzzer-4838195726123008
Fixes: 26384/clusterfuzz-testcase-minimized-ffmpeg_dem_G729_fuzzer-5173450337157120
Fixes: 26396/clusterfuzz-testcase-minimized-ffmpeg_dem_PCM_S24BE_fuzzer-5071092206796800

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

libavformat/asfdec_f.c

@@ -607,6 +607,8 @@ static int asf_read_ext_stream_properties(AVFormatContext *s, int64_t size)
         ff_get_guid(pb, &g);
         size = avio_rl16(pb);
         ext_len = avio_rl32(pb);
+        if (ext_len < 0)
+            return AVERROR_INVALIDDATA;
         avio_skip(pb, ext_len);
         if (stream_num < 128 && i < FF_ARRAY_ELEMS(asf->streams[stream_num].payload)) {
             ASFPayload *p = &asf->streams[stream_num].payload[i];