voc_packet: validate sample_rate

A negative sample rate doesn't make sense and triggers assertions in
av_rescale_rnd.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/voc_packet.c

@@ -106,6 +106,11 @@ ff_voc_get_packet(AVFormatContext *s, AVPacket *pkt, AVStream *st, int max_size)
         }
     }
 
+    if (par->sample_rate <= 0) {
+        av_log(s, AV_LOG_ERROR, "Invalid sample rate %d\n", par->sample_rate);
+        return AVERROR_INVALIDDATA;
+    }
+
     if (tmp_codec >= 0) {
         tmp_codec = ff_codec_get_id(ff_voc_codec_tags, tmp_codec);
         if (par->codec_id == AV_CODEC_ID_NONE)