[net, trivial] explicitly use std::string for FindNode

- in OpenNetworkConnection we have a FindNode call, which gets a const
  char*, wheras we only have defined a FindNode for std::string

src/net.cpp

@@ -1363,7 +1363,7 @@ bool OpenNetworkConnection(const CAddress& addrConnect, CSemaphoreGrant *grantOu
             FindNode((CNetAddr)addrConnect) || CNode::IsBanned(addrConnect) ||
             FindNode(addrConnect.ToStringIPPort()))
             return false;
-    } else if (FindNode(pszDest))
+    } else if (FindNode(std::string(pszDest)))
         return false;
 
     CNode* pnode = ConnectNode(addrConnect, pszDest);