[Security] Subclasses of MessageHandler implicitly accept all messages, validate method insufficient for use.

[Gamebuster19901]

Describe the bug
The following method is not abstract:

litiengine/src/de/gurkenlabs/litiengine/net/messages/MessageHandler.java

Lines 45 to 47 in 649ceea

	protected boolean validate(final T message) {
	return message != null;
	}

This means that a subclass of ClientMessage will accept any message (as long as it's not null) by default if the developer forgets to implement validate.

Additionally, this method needs an address and port field to validate the client is the one who actually sent the message.

Expected behavior
Subclasses of MessageHandler should be forced to implement a validate method to ensure that the message is valid, and the message actually came from the expected client.

Your System:

• LITIENGINE version: [e.g. 0.5.0-beta]