Check number of stars in custom system

This is supposed to fix the (guessed) reason of the assertion in #2718. It will check that the number of stars defined in a (non-random) custom system matches the number of star types given in the ```CustomSystem:new``` call. This is important, because this is used internally as the number of stars in the system.

@walterar: Please check if that fixes your issue by finding your invalid custom system.