webp: Make sure enough bytes are available

Every chunk needs at least 8 bytes for chunk_type and chunk_size.
Prevent a possible infinite loop.

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/webp.c

@@ -1362,7 +1362,7 @@ static int webp_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
         return AVERROR_INVALIDDATA;
     }
 
-    while (bytestream2_get_bytes_left(&gb) > 0) {
+    while (bytestream2_get_bytes_left(&gb) > 8) {
         char chunk_str[5] = { 0 };
 
         chunk_type = bytestream2_get_le32(&gb);