check that duration is set to avoid fpe

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20457 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/mov.c

@@ -1500,6 +1500,7 @@ static void mov_build_index(MOVContext *mov, AVStream *st)
                 }
             }
         }
+        if (st->duration > 0)
             st->codec->bit_rate = stream_size*8*sc->time_scale/st->duration;
     } else {
         for (i = 0; i < sc->chunk_count; i++) {