Stop handling the current packet if the server is shutting down

Previously, terminating a running server from the terminal would
sometimes print a benign warning message if the server was currently
handling an incoming packet. This happened because CNServer::step()
would continue handling the packet after CNServer::kill() released the
activeCrit mutex. Now it first re-checks if active has been set to false
in the mean time after acquiring the mutex.

src/core/CNProtocol.cpp

@@ -473,6 +473,10 @@ void CNServer::start() {
             } else {
                 std::lock_guard<std::mutex> lock(activeCrit); // protect operations on connections
 
+                // halt packet handling if server is shutting down
+                if (!active)
+                    return;
+
                 // player sockets
                 if (connections.find(fds[i].fd) == connections.end()) {
                     std::cout << "[WARN] Event on non-existant socket?" << std::endl;