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>