This matches production code which also zeros these buffers
Fixes: use of uninitialized values
Fixes: 70885/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VP6F_fuzzer-4610946029387776 (and likely others)
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>