rv10: check that extradata is large enough

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC:libav-stable@libav.org

libavcodec/rv10.c

@@ -346,6 +346,11 @@ static int rv20_decode_picture_header(RVDecContext *rv)
         f = get_bits(&s->gb, rpr_bits);
 
         if (f) {
+            if (s->avctx->extradata_size < 8 + 2 * f) {
+                av_log(s->avctx, AV_LOG_ERROR, "Extradata too small.\n");
+                return AVERROR_INVALIDDATA;
+            }
+
             new_w = 4 * ((uint8_t*)s->avctx->extradata)[6 + 2 * f];
             new_h = 4 * ((uint8_t*)s->avctx->extradata)[7 + 2 * f];
         } else {