Log a critical error message if a node is WFC, but its socket is still in use

This should avoid/work-around issue #630

Nelgin's terminal server log indicated that the node's configuration was being
reloaded for a new client connection (which doesn't normally/always happen,
only when there's been a config file change or a node "rerun" requested),
while there was still a user online using that node. This would explain the
segfault (configuration fields being freed and reallocated).

The node_socket[] array indicates when a node thread is actively servicing
a client, so it should contain INVALID_SOCKET for the any node's that WFC.
If it doesn't, that means something/someone changed the node's status value
to 0 (WFC) in the node.dab file while the node was still in-use. Let's not
crash in this case and rather just log the message and give the client the
next available node (if any).