avformat/oggparsetheora: clip duration within 64bit

Fixes: signed integer overflow: 9079256848778919936 - -288230376151711746 cannot be represented in type 'long'
Fixes: 58248/clusterfuzz-testcase-minimized-ffmpeg_dem_OGG_fuzzer-6326851353313280

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

libavformat/oggparsetheora.c

@@ -196,7 +196,7 @@ static int theora_packet(AVFormatContext *s, int idx)
         if(s->streams[idx]->start_time == AV_NOPTS_VALUE && os->lastpts != AV_NOPTS_VALUE) {
             s->streams[idx]->start_time = os->lastpts;
             if (s->streams[idx]->duration > 0)
-                s->streams[idx]->duration -= s->streams[idx]->start_time;
+                s->streams[idx]->duration = av_sat_sub64(s->streams[idx]->duration, s->streams[idx]->start_time);
         }
     }