debug-on-entry with empty functions.

[Lute Kamstra]

The Lisp interpreter doesn't seem to mind when I define a function
with an empty body:

  (defun some-fun ())

Calling it simply returns nil.  However, when I do:

  (debug-on-entry 'some-fun)

I get:

Debugger entered--Lisp error: (wrong-type-argument consp nil)
  debug-on-entry-1(some-fun (lambda nil) t)
  debug-on-entry(some-fun)
  eval((debug-on-entry (quote some-fun)))
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp)

Probably either the Lisp interpreter should signal an error on
defining a function with an empty body or debug-on-entry should take
this case into account.

Lute.