vkr: validate array sizes and string lengths
This is generated from https://gitlab.freedesktop.org/olv/venus-protocol/-/merge_requests/17.
For a (fooCount, pFoo) array, we encode fooCount twice. Previously, we
used one for allocation and the other for initialization. When the two
differed, we could allocate an array of N elements but initialize only
the first M elements. We only validated that M <= N.
After this commit, vn_decode_array_size validates that M == N.
The other main change is that this commit adds vn_decode_char_array to
make sure strings are null-terminated.