avcodec/wnv1: More strict buffer size check

This requires at least 25% of a picture to allocate and decode it

Fixes: Timeout
Fixes: 1845/clusterfuzz-testcase-minimized-5075974343360512

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