net: ipv6: Fixed IPv6 string to address conversion off-by-one error

One extra character was being checked in the IPv6 string which caused the
last character of the address to be neither '\0' nor ':'. This raises an
error condition and causes the function to always return an error. This
issue was resolved by this fix.

Signed-off-by: Ehsan Mohandesi <emohandesi@microsoft.com>
Reviewed-by: Viacheslav Mitrofanov <v.v.mitrofanov@yadro.com>