Throw a TypeError if the extends expression does not evaluate to a Class

If the `extends`/super expression does not evaluate to a Class-instance, slash still tries to use it as a `sl_class_t` type, which results in a segmentation fault, when it is used.

Example:

    >> class Test extends 1 {}
    Segmentation fault

This PR just adds a type-check before the `vsuper` value is actually used.