lavf: fix arithmetic overflows in avformat_seek_file()

The values compared here can be more than INT64_MAX apart.  Since the
difference is always positive, converting to uint64_t before subtracting
gives the correct result without overflows.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/utils.c

@@ -1762,7 +1762,7 @@ int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts, int
     //Fallback to old API if new is not implemented but old is
     //Note the old has somewat different sematics
     if(s->iformat->read_seek || 1)
-        return av_seek_frame(s, stream_index, ts, flags | (ts - min_ts > (uint64_t)(max_ts - ts) ? AVSEEK_FLAG_BACKWARD : 0));
+        return av_seek_frame(s, stream_index, ts, flags | ((uint64_t)ts - min_ts > (uint64_t)max_ts - ts ? AVSEEK_FLAG_BACKWARD : 0));
 
     // try some generic seek like seek_frame_generic() but with new ts semantics
 }