QSafeStateTransition::statePropertyChange needs boundary checks or to be private method

Currently QSafeStateTransition::statePropertyChange() method is used only within the class but it is a public method in that class. Either add boundary checks for the index parameter, or move this function to be private so that it's only usable from this one class.

Or add both, boundary checks and move it to private.

This method should throw out of bounds exception when the fetched index is out of bounds. This would be in line with other similar index-fetching methods.