avformat/subviewerdec: fail on AV_NOPTS_VALUE

Such values are not supported by ff_subtitles_queue*

Fixes: signed integer overflow: 10 - -9223372036854775808 cannot be represented in type 'long'
Fixes: 24193/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5714901855895552

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

libavformat/subviewerdec.c

@@ -148,6 +148,10 @@ static int subviewer_read_header(AVFormatContext *s)
             new_event = 1;
             pos = avio_tell(s->pb);
         } else if (*line) {
+            if (pts_start == AV_NOPTS_VALUE) {
+                res = AVERROR_INVALIDDATA;
+                goto end;
+            }
             if (!new_event) {
                 sub = ff_subtitles_queue_insert(&subviewer->q, "\n", 1, 1);
                 if (!sub) {