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

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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)))))