Classlib: Prevent possible infinite recursion in *initClassTree

[jamshark70]

Purpose and Motivation

Fixes #5574 by moving classesInited.add(aClass); to precede the potentially recursive call.

This test case does hang the interpreter without the fix, and doesn't hang it with the fix.

Kaboom {
	*initClass {
		Class.initClassTree(this);
	}
}

Types of changes

• Bug fix

To-do list

• Code is tested
• All tests are passing
• This PR is ready for review

I can't imagine a good way to unit-test this.