Add a one-word sentinel value of 0x0 at the end of each buf_t chunk

This helps protect against bugs where any part of a buf_t's memory
is passed to a function that expects a NUL-terminated input.

It also closes TROVE-2016-10-001 (aka bug 20384).
buf_get_default_chunk_size(const buf_t *buf)
{
  return buf->default_chunk_size;
}

buf_get_default_chunk_size(const buf_t *buf)
{
  return buf->default_chunk_size;
}
