Prevent unspecified behaviour when zeroing union.
Prevent unspecified behaviour when zeroing union.

The C standard (§J.1-1) reads:
> The following [is] unspecified
> — The value of a union member other than the last one stored into (6.2.6.1).

Zero-ing the `raw` member first makes the value of the `nh` member
unspecified. It’s better to zeroise the whole union.

jech/babeld,jech/babeld