bug#54478: unwanted source references

[Matt Wette]

I think the error is in use of "src" arg to make-seq in the following loop,
in ice-9/psyntax.scm, starting at line 1633:

                (let lp ((var-ids var-ids) (vars vars) (vals vals)
                         (tail (expand-tail-expr)))
                  (cond
                   ((null? var-ids) tail)
                   ((not (car var-ids))
                    (lp (cdr var-ids) (cdr vars) (cdr vals)
                        (make-seq src ((car vals)) tail)))
                   (else
                    (let ((var-ids (map (lambda (id)
                                          (if id (syntax->datum id) '_))
                                        (reverse var-ids)))
                          (vars (map (lambda (var) (or var (gen-label)))
                                     (reverse vars)))
                          (vals (map (lambda (expand-expr id)
                                       (if id
                                           (expand-expr)
                                           (make-seq src
                                                     (expand-expr)
                                                     (build-void src))))
                                     (reverse vals) (reverse var-ids))))
                      (build-letrec src #t var-ids vars vals tail)))))