avformat/utils: Check cur_dts in update_initial_timestamps() more

Fixes: runtime error: signed integer overflow: 18133149658382192 - -9223090561878065151 cannot be represented in type 'long long'
Fixes: crbug 831552
Reported-by: Matt Wolenetz <wolenetz@google.com>
Reviewed-by: Matt Wolenetz <wolenetz@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/utils.c

@@ -1129,6 +1129,7 @@ static void update_initial_timestamps(AVFormatContext *s, int stream_index,
     if (st->first_dts != AV_NOPTS_VALUE ||
         dts           == AV_NOPTS_VALUE ||
         st->cur_dts   == AV_NOPTS_VALUE ||
+        st->cur_dts < INT_MIN + RELATIVE_TS_BASE ||
         is_relative(dts))
         return;