avformat/oggparsetheora: Check duration to be not AV_NOPTS_VALUE (and positive) before use

Fixes: 1536b9b096a8f95b742bae9d3d761cc6/signal_sigsegv_294aaed_7322_4fad88a38dc8952dc20dcb60c1895758.ogg

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/oggparsetheora.c

@@ -191,7 +191,7 @@ static int theora_packet(AVFormatContext *s, int idx)
         os->lastpts = os->lastdts   = theora_gptopts(s, idx, os->granule, NULL) - duration;
         if(s->streams[idx]->start_time == AV_NOPTS_VALUE) {
             s->streams[idx]->start_time = os->lastpts;
-            if (s->streams[idx]->duration)
+            if (s->streams[idx]->duration > 0)
                 s->streams[idx]->duration -= s->streams[idx]->start_time;
         }
     }