initialization of userdata

[Dirk Laurie <dirk.laurie@...>]

The manual says of `luanewuserdata`:
…allocates a new block of memory with the given size, pushes onto the
stack a new full userdata with the block address, and returns this
address.

It does not say the block is initialized to zero, but that invariably
has so far been true on my system.

Implementation detail?