avcodec/webp: Update canvas size in vp8_lossy_decode_frame() as in vp8_lossless_decode_frame()

Fixes: 1407/clusterfuzz-testcase-minimized-6044604124102656
Fixes: 1420/clusterfuzz-testcase-minimized-6059927359455232

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