avcodec/webp: Check ref_x/y

Fixes: 70991/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WEBP_fuzzer-5544067620995072
Fixes: use of uninintailized value

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 7c1e732ad2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/webp.c

@@ -704,6 +704,9 @@ static int decode_entropy_coded_image(WebPContext *s, enum ImageRole role,
             ref_x = FFMAX(0, ref_x);
             ref_y = FFMAX(0, ref_y);
 
+            if (ref_y == y && ref_x >= x)
+                return AVERROR_INVALIDDATA;
+
             /* copy pixels
              * source and dest regions can overlap and wrap lines, so just
              * copy per-pixel */