resolve.c: fix valgrind 'uninitialized memory' warning

resolve.c: fix valgrind 'uninitialized memory' warning

fix the following warning generated by valgrind if ipv6 is enabled:

Syscall param write(buf) points to uninitialised byte(s)
   at 0x4000982: (within /lib/ld-2.9.so)
   by 0x80681A8: Resolve_Name (resolve.c:477)
   by 0x805439F: Conn_Handler (conn.c:1658)
   by 0x804AA7C: main (ngircd.c:331)

The warning is because ng_ipaddr_t can be a union, and only the
necessary parts are initialised.  The callers know what part
of the union is valid, so this is not a bug.