_msgpack_buffer_add_new_chunk zero-out the newly allocated tail

[casperisfine]

Fix: #342
Ref: #341

Reseting the memory in _msgpack_buffer_alloc_new_chunk was pointless because the previous tail is immediately copied into it, and it's the tail that is then used by the caller. So it's the tail we should have zeroed-out.

I think what is happening is in _msgpack_buffer_expand:

    if(b->tail.mapped_string != NO_MAPPED_STRING || capacity <= MSGPACK_RMEM_PAGE_SIZE) {
        /* allocate new chunk */
        _msgpack_buffer_add_new_chunk(b);

        char* mem = _msgpack_buffer_chunk_malloc(b, &b->tail, length, &capacity);

        char* last = mem;
        if(data != NULL) {
            memcpy(mem, data, length);
            last += length;
        }

        /* rebuild tail chunk */
        b->tail.first = mem;
        b->tail.last = last;
        b->tail.mapped_string = NO_MAPPED_STRING;
        b->tail_buffer_end = mem + capacity;

_msgpack_buffer_add_new_chunk potentially leave b->tail.mapped_string as a garbage reference, then _msgpack_buffer_chunk_malloc may trigger a GC.

b->tail.mapped_string is only reset later.

I'm still trying to craft a repro for such error case.