bug#19699: 25.0.50; lexical-let + mapcar cause infloop

[Katsumi Yamaoka]

A Lisp form that uses `lexical-let' and `mapcar' (or `mapc'),
like this, cannot be evaluated nor byte-compiled because it
looks like causing an infinite loop[1].

(defun testing ()
  (lexical-let (var)
    (mapcar #'identity '("Hello" "World"))))

[1] There is no means to stop infloop on Cygwin Emacs, so I'm
  hard to debug it.

In GNU Emacs 25.0.50.1 (i686-pc-cygwin, GTK+ Version 3.10.9)
 of 2015-01-27 on localhost
Windowing system distributor `The Cygwin/X Project', version 11.0.11501000
Configured using:
 `configure --verbose --with-x-toolkit=gtk3'