Verify that we won't call a method on a non-existant element.

This seems to be the root of a handfull of crashes.

stepmania/src/NetworkSyncServer.cpp

@@ -515,6 +515,7 @@ void StepManiaLanServer::SendStatsToClients()
 
 void StepManiaLanServer::SendNetPacket(const unsigned int client, PacketFunctions& Packet)
 {
+	if ( client < Client.size() )
 		Client[client]->clientSocket.SendPack((char*)Packet.Data, Packet.Position);
 }
 
@@ -592,6 +593,7 @@ void StepManiaLanServer::NewClientCheck()
 
 void StepManiaLanServer::SendValue(uint8_t value, const unsigned int clientNum)
 {
+	if ( clientNum < Client.size() )
 		Client[clientNum]->clientSocket.SendPack((char*)&value, sizeof(uint8_t));
 }
 
@@ -814,6 +816,7 @@ void StepManiaLanServer::ServerChat(const CString& message)
 bool StepManiaLanServer::CheckConnection(const unsigned int clientNum)
 {
 	//If there is an error close the socket.
+	if ( clientNum < Client.size() )
 		if (Client[clientNum]->clientSocket.IsError())
 		{
 			Disconnect(clientNum);