Zero-out msgpack_buffer_chunk_t after allocation

[casperisfine]

Fix: https://github.com/msgpack/msgpack-ruby/issues/341

These struct contain a VALUE reference so if we don't zero it out, it could be pointing at a T_NONE or some other old object slot.

Especially since we can re-use existing chunks.

cc @peterzhu2118 @pavel-workato