Bug fix: LIAR previously miscompiled local procedures with optional
parameters if they were never passed.

Formerly MIT Scheme used unassigned reference traps for optional
parameters without values; now it uses a #!default object.  But LIAR
hadn't gotten the memo, and if every call site omitted the argument,
LIAR would constant-fold an unassigned reference trap instead of
#!default.

(let ()
  (define (test #!optional x)
    (pp (map-reference-trap (lambda () x))))
  (test))
