brstm: reject negative sample rate

A negative sample rate causes assertion failures in av_rescale_rnd.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/brstm.c

@@ -205,7 +205,7 @@ static int read_header(AVFormatContext *s)
     avio_skip(s->pb, 1); // padding
 
     st->codec->sample_rate = bfstm ? read32(s) : read16(s);
-    if (!st->codec->sample_rate)
+    if (st->codec->sample_rate <= 0)
         return AVERROR_INVALIDDATA;
 
     if (!bfstm)