avformat/id3v2: Check taglen in read_uslt()

Fixes: Timeout (read mostly the same data repeatly)
Fixes: 52457/clusterfuzz-testcase-minimized-ffmpeg_dem_ALP_fuzzer-6610706313379840
Fixes: 53098/clusterfuzz-testcase-minimized-ffmpeg_dem_SOL_fuzzer-6481382981632000

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

libavformat/id3v2.c

@@ -377,10 +377,10 @@ static void read_uslt(AVFormatContext *s, AVIOContext *pb, int taglen,
     lang[3] = '\0';
     taglen -= 3;
 
-    if (decode_str(s, pb, encoding, &descriptor, &taglen) < 0)
+    if (decode_str(s, pb, encoding, &descriptor, &taglen) < 0 || taglen < 0)
         goto error;
 
-    if (decode_str(s, pb, encoding, &text, &taglen) < 0)
+    if (decode_str(s, pb, encoding, &text, &taglen) < 0 || taglen < 0)
         goto error;
 
     // FFmpeg does not support hierarchical metadata, so concatenate the keys.