avcodec/webp: Check available space in loop in decode_entropy_coded_image()

Fixes: Timeout
Fixes: 35401/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WEBP_fuzzer-5714401821851648

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/webp.c

@@ -627,6 +627,9 @@ static int decode_entropy_coded_image(WebPContext *s, enum ImageRole role,
     while (y < img->frame->height) {
         int v;
 
+        if (get_bits_left(&s->gb) < 0)
+            return AVERROR_INVALIDDATA;
+
         hg = get_huffman_group(s, img, x, y);
         v = huff_reader_get_symbol(&hg[HUFF_IDX_GREEN], &s->gb);
         if (v < NUM_LITERAL_CODES) {