This will probably make no functional difference, but will squash a
warning from scan-build:
```
../../../../source/glib/glib/gmessages.c:2243:42: warning: The left operand of '==' is a garbage value [core.UndefinedBinaryOperatorResult]
  if (err == 0 && addr.storage.ss_family == AF_UNIX)
                  ~~~~~~~~~~~~~~~~~~~~~~ ^
```

It seems like a reasonable thing to warn about. Initialising the full
union to zero should avoid any possibility of undefined behaviour like
that.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #1767