Fix for nested invocations of call-with-values

[donaldsonjw]

To properly handle nested call-with-values invocations where the outer call-with-values generator does not return multiple values via values, we reset the default number of return values to 1 before invoking the outer consumer.

For example, we want

(call-with-values (lambda ()
                   (call-with-values (lambda () (values 1 2 3))
                                     +))
                   list)

to return (6) not (6 2 3)

[manuel-serrano]

Thank you Joseph for this useful fix. All my apologizes for the latency of my reaction.