Use-of-uninitialized-value in cdef_filter_block_c
OSS-Fuzz report:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=49787
```
Crash type: Use-of-uninitialized-value
Crash state:
cdef_filter_block_c
cdef_filter_block_8x8_c
dav1d_cdef_brow_8bpc
```