avformat/utils: Check dts - (1<<pts_wrap_bits) overflow

Fixes: signed integer overflow: -9223372036842389247 - 2147483648 cannot be represented in type 'long long'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_FLV_fuzzer-4845007531671552

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

libavformat/utils.c

@@ -1254,7 +1254,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
         presentation_delayed = 1;
 
     if (pkt->pts != AV_NOPTS_VALUE && pkt->dts != AV_NOPTS_VALUE &&
-        st->pts_wrap_bits < 63 &&
+        st->pts_wrap_bits < 63 && pkt->dts > INT64_MIN + (1LL << (st->pts_wrap_bits - 1)) &&
         pkt->dts - (1LL << (st->pts_wrap_bits - 1)) > pkt->pts) {
         if (is_relative(st->cur_dts) || pkt->dts - (1LL<<(st->pts_wrap_bits - 1)) > st->cur_dts) {
             pkt->dts -= 1LL << st->pts_wrap_bits;