avcodec/webp: Fixes null pointer dereference

Fixes: 1470/clusterfuzz-testcase-minimized-5404421666111488
Fixes: 1472/clusterfuzz-testcase-minimized-5677426430443520
Fixes: 1875/clusterfuzz-testcase-minimized-5536474562822144

Approved-by: BBB

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