Do not release TCP sockets while still in use

SDLNet_TCP_Close() not only closes, but also release the resources.

Therefore, we must not close the socket if another thread attempts to
read it.

For that purpose, move socket closing from server_stop() to
server_destroy().

app/src/server.h

@@ -31,4 +31,7 @@ TCPsocket server_connect_to(struct server *server, const char *serial);
 // disconnect and kill the server process
 void server_stop(struct server *server, const char *serial);
 
+// close and release sockets
+void server_destroy(struct server *server);
+
 #endif