[parenscript-devel] NESTING-DEPTH crashes on improper lists

This causes compilations of forms like the following to fail:

(ps (defun foo (a) (when a (destructuring-bind (b . c) bar (list b c)))))

=> TYPE-ERROR

Backtrace: 0: (NESTING-DEPTH (B . C)) 1: (NESTING-DEPTH (DESTRUCTURING-BIND (B . C) BAR (LIST B C))) 2: (NESTING-DEPTH (PROGN (DESTRUCTURING-BIND (B . C) BAR (LIST B C)))) 3: (NESTING-DEPTH (IF A (PROGN (DESTRUCTURING-BIND (B . C) BAR (LIST B C)))))