avformat/nutdec: Check fields

Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 50993/clusterfuzz-testcase-minimized-ffmpeg_dem_NUT_fuzzer-6566001610719232

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

libavformat/nutdec.c

@@ -245,6 +245,11 @@ static int decode_main_header(NUTContext *nut)
     for (i = 0; i < 256;) {
         int tmp_flags  = ffio_read_varlen(bc);
         int tmp_fields = ffio_read_varlen(bc);
+        if (tmp_fields < 0) {
+            av_log(s, AV_LOG_ERROR, "fields %d is invalid\n", tmp_fields);
+            ret = AVERROR_INVALIDDATA;
+            goto fail;
+        }
 
         if (tmp_fields > 0)
             tmp_pts = get_s(bc);