avformat/id3v2: read_uslt() check for the amount read

Fixes: timeout
Fixes: 66783/clusterfuzz-testcase-minimized-ffmpeg_dem_GENH_fuzzer-5356884892647424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c0f4abe2aa)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/id3v2.c

@@ -366,7 +366,7 @@ static void read_uslt(AVFormatContext *s, AVIOContext *pb, int taglen,
     int encoding;
     int ok = 0;
 
-    if (taglen < 1)
+    if (taglen < 4)
         goto error;
 
     encoding = avio_r8(pb);