net: guard remaining free-function g_connman dereferences (AddressCurrentlyConnected) (follow-up to #2558)

Follow-up to #2558.

Gap

Most external g_connman consumers use the established if (g_connman) guard (covering the early-startup / shutdown-teardown window where the unique_ptr is null). One free function still dereferences it unguarded:

• AddressCurrentlyConnected(const CService& addr) — src/net.cpp:337 — calls g_connman->GetAddrMan().Connected(addr) with no guard. Reachable during the shutdown window after g_connman.reset().

Fix

Add the if (g_connman) guard (or route the call through a CConnman member). While here, audit for any other unguarded free-function g_connman dereferences.

Severity

LOW — shutdown-window only; the window is narrow. Good-first-issue-sized hardening.

Note: the other site flagged in the stack review (ConnectNode) is already resolved — it was promoted to a CConnman member in the #2558 stack, so it no longer dereferences g_connman as a free function.