avformat/utils: Check dts in update_initial_timestamps() more

Fixes: signed integer overflow: -9223372036853488158 - 90000000 cannot be represented in type 'long long'
Fixes: 26910/clusterfuzz-testcase-minimized-ffmpeg_dem_MPSUB_fuzzer-6696625298866176

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

@@ -1106,6 +1106,7 @@ static void update_initial_timestamps(AVFormatContext *s, int stream_index,
         dts           == AV_NOPTS_VALUE ||
         st->cur_dts   == AV_NOPTS_VALUE ||
         st->cur_dts < INT_MIN + RELATIVE_TS_BASE ||
+        dts  < INT_MIN + (st->cur_dts - RELATIVE_TS_BASE) ||
         is_relative(dts))
         return;