mov: Don't use a negative duration for setting other fields

Some files have the duration set to -1 in the mdhd atom, more
or less legitimately. (We produce such files ourselves, for the
initial duration in fragmented mp4 files.)

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavformat/mov.c

@@ -2130,7 +2130,7 @@ static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                                              ((double)st->codec->width * sc->height), INT_MAX);
         }
 
-        if (st->duration != AV_NOPTS_VALUE)
+        if (st->duration != AV_NOPTS_VALUE && st->duration > 0)
             av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                       sc->time_scale*st->nb_frames, st->duration, INT_MAX);
     }
@@ -2863,7 +2863,7 @@ static int mov_read_header(AVFormatContext *s)
         for (i = 0; i < s->nb_streams; i++) {
             AVStream *st = s->streams[i];
             MOVStreamContext *sc = st->priv_data;
-            if (st->duration)
+            if (st->duration > 0)
                 st->codec->bit_rate = sc->data_size * 8 * sc->time_scale / st->duration;
         }
     }