Do not allow 1:2:3:4:5:6:7:8::9 as IPv6 address

The Net_IPv6 code has a bug - checkIPv6 returns "true" for addresses like:
1::2:3:4:5:6:7:8:9
1:2::3:4:5:6:7:8:9
...
1:2:3:4:5:6:7::8:9
1:2:3:4:5:6:7:8::9
these also emit the message:
Warning: str_repeat(): Second argument has to be greater than or equal to 0 in /etc/inc/IPv6.inc on line 496

It is fixed by making uncompress() always insert at least one ":0:" for any "::" in the address - a "::" always implies at least ":0:". This removes the str_repeat() warning, and means that the uncompressed IPv6 address has >8 numbers in it, so it fails the validity check (as it should).