avoid delivering 0 values to 1-valued continuations in srfi-19

avoid delivering 0 values to 1-valued continuations in srfi-19

* module/srfi/srfi-19.scm: Some parts of this code used a strange idiom,
  `(values)', to indicate that a procedure did nothing. However, quoth
  R5RS:

     Except for continuations created by the `call-with-values'
     procedure, all continuations take exactly one value.

  Indeed the VM indicated this error. I reworked the code to avoid these
  cases.