fix for (apply values '(1))

fix for (apply values '(1))

* libguile/vm-i-system.c (return/values): In the
  multiple-values-to-a-single-value-continuation (or MV but where N=1),
  null out the correct number of values from the stack. Fixes aborts on
  (apply values '(1)).

* testsuite/t-values.scm (call-with-values): Add a test.