[net/net processing] check banman pointer before dereferencing

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.

src/net_processing.h

@@ -29,6 +29,7 @@ static const int DISCOURAGEMENT_THRESHOLD{100};
 class PeerLogicValidation final : public CValidationInterface, public NetEventsInterface {
 private:
     CConnman* const connman;
+    /** Pointer to this node's banman. May be nullptr - check existence before dereferencing. */
     BanMan* const m_banman;
     ChainstateManager& m_chainman;
     CTxMemPool& m_mempool;