Fix some bad IPv6 validation issues

Following bad addresses are being validated as good by checkIPv6()

1:2:3:4:5:6:7:8:1.2.3.4
::1:2:3:4:5:6:7:8
1:2:3:4:5:6:7:8::
1:2:3:4:5:6:::8
::::a
::::
1::2::3

Obtained from: pfSense
MFH: 2016Q3
Sponsored by: Rubicon Communications (Netgate)