Merge #13776: net: Add missing verification of IPv6 address in CNetAddr::GetIn6Addr(...)

Summary:
2fb0066b9932f3725a5a0533530453c6da4010ce net: Add missing verification of IPv6 address in CNetAddr::GetIn6Addr(...) (practicalswift)

Pull request description:

Add missing verification of IPv6 address in `CNetAddr::GetIn6Addr(...)`.

Tree-SHA512: 8b0681252eec9cf293a2043e99fdacec520e321b477d8aca1cbd6327b85bf6c5e8cd820fb914f097c62655947e88745ebccd824a78b995a8186e910e6fe520aa

Backport of Core PR13776
https://github.com/bitcoin/bitcoin/pull/13776/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4772