mvdec: 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/mvdec.c

@@ -317,6 +317,10 @@ static int mv_read_header(AVFormatContext *avctx)
         ast->codecpar->codec_type  = AVMEDIA_TYPE_AUDIO;
         ast->nb_frames          = vst->nb_frames;
         ast->codecpar->sample_rate = avio_rb32(pb);
+        if (ast->codecpar->sample_rate <= 0) {
+            av_log(avctx, AV_LOG_ERROR, "Invalid sample rate %d\n", ast->codecpar->sample_rate);
+            return AVERROR_INVALIDDATA;
+        }
         avpriv_set_pts_info(ast, 33, 1, ast->codecpar->sample_rate);
         if (set_channels(avctx, ast, avio_rb32(pb)) < 0)
             return AVERROR_INVALIDDATA;