With-Open-File :if-does-not-exist nil

[Rob MacLachlan <RAM@CMU-CS-C.ARPA>]

    What should be done in With-Open-File if open returns NIL as a
consequence of a :if-does-not-exist or :if-exists or :probe keyword?
Our current implementation simply binds the result, whatever it is, to
the variable.  It has been suggested that it would be better to not
evaluate the body in this case.  I have my doubts about this, but the
current implementation also has problems, since it will attempt to
close NIL when the body is exited.

  Rob