The write_vtable() function's comment claimed to "fill the WIP vtable
with zeros" but make_space() only reserves memory without initializing
it. When using custom allocators with non-zeroed buffers, unset vtable
field entries would contain garbage instead of zero (which indicates
"use default value").
This fix explicitly zeros the vtable memory after reserving space,
matching the C++ implementation's buf_.fill_big() behavior.
Added regression test using a garbage-filled allocator (0xAA) that
verifies vtable entries for unset fields are properly zeroed.
Fixes#8894
