net: only delete CConnman if it's been created

In the case of (for example) an already-running bitcoind, the shutdown sequence
begins before CConnman has been created, leading to a null-pointer dereference
when g_connman->Stop() is called.

Instead, Just let the CConnman dtor take care of stopping.

src/net.cpp

@@ -2205,6 +2205,7 @@ void CConnman::DeleteNode(CNode* pnode)
 
 CConnman::~CConnman()
 {
+    Stop();
 }
 
 size_t CConnman::GetAddressCount() const