[net/net processing] check banman pointer before dereferencing
Summary:
Although we currently don't do this, it should be possible to create a CConnman or PeerLogicValidation without a Banman instance. Therefore always check that banman exists before dereferencing the pointer. Also add comments to the m_banman members of CConnman and PeerLogicValidation to document that these may be nullptr.
Backport of core PR19514.
Depends on D8794.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D8795