vp3: Check the framerate for validity

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

libavcodec/vp3.c

@@ -2199,6 +2199,10 @@ static int theora_decode_header(AVCodecContext *avctx, GetBitContext *gb)
     fps.num = get_bits_long(gb, 32);
     fps.den = get_bits_long(gb, 32);
     if (fps.num && fps.den) {
+        if (fps.num < 0 || fps.den < 0) {
+            av_log(avctx, AV_LOG_ERROR, "Invalid framerate\n");
+            return AVERROR_INVALIDDATA;
+        }
         av_reduce(&avctx->time_base.num, &avctx->time_base.den,
                   fps.den, fps.num, 1<<30);
     }