r3d: fix division by 0 with 0 sample rate

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/r3d.c

@@ -312,6 +312,7 @@ static int r3d_read_reda(AVFormatContext *s, AVPacket *pkt, Atom *atom)
 
     pkt->stream_index = 1;
     pkt->dts = dts;
+    if (st->codec->sample_rate)
         pkt->duration = av_rescale(samples, st->time_base.den, st->codec->sample_rate);
     av_dlog(s, "pkt dts %"PRId64" duration %d samples %d sample rate %d\n",
             pkt->dts, pkt->duration, samples, st->codec->sample_rate);