bug#24911: 25.1; Emacs hangs or crashes on use of read syntax for circular objects in IELM

[Gemini Lasswell]

Typing an example from the "Read Syntax for Circular Objects" section of
the manual into IELM can cause Emacs to hang or crash. I can reproduce
this fairly consistently with an Emacs built from master on my machine. It
took me several tries, but I was also able to reproduce it with the Emacs
25.1 installed by homebrew. Here are the steps:

emacs -Q
M-x ielm RET
#1=(a #1#) RET
C-p C-p RET

Result: (on OS X) rainbow spinning mouse cursor, unresponsive Emacs

There may be something timing-related going on, because these steps
don't work every time and they seem to work more consistently if I enter
the sequence of keystrokes quickly. If it doesn't happen the first time,
navigating again to a previous line and editing it by adding a quote
somewhere or another symbol and pressing RET again may cause it to
happen.

When I encountered this bug, I was working on one of my own branches
based on commit eb610f2 in master, and Emacs crashed and gave me a stack
trace, which I've attached. Since then I have not been able to reproduce
the crash, just the hang.

emacs-crash.txt
Description: Text document

In GNU Emacs 25.1.1 (x86_64-apple-darwin15.6.0, NS appkit-1404.47 Version 
10.11.6 (Build 15G1004))
 of 2016-10-09 built on rainbow.local
Windowing system distributor 'Apple', version 10.3.1404
Configured using:
 'configure --disable-dependency-tracking --disable-silent-rules
 --enable-locallisppath=/usr/local/share/emacs/site-lisp
 --infodir=/usr/local/Cellar/emacs/25.1/share/info/emacs
 --prefix=/usr/local/Cellar/emacs/25.1 --without-x --with-xml2
 --without-dbus --with-gnutls --with-imagemagick --with-rsvg --with-ns
 --disable-ns-self-contained'

Configured features:
JPEG RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS
NS

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired format-spec rfc822 mml
mml-sec password-cache epg epg-config gnus-util mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util help-fns help-mode easymenu
cl-loaddefs pcase cl-lib mail-prsvr mail-utils time-date mule-util
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel ns-win ucs-normalize term/common-win tool-bar dnd fontset image
regexp-opt fringe tabulated-list newcomment elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow timer select scroll-bar
mouse jit-lock font-lock syntax facemenu font-core frame cl-generic cham
georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote kqueue cocoa ns multi-tty
make-network-process emacs)

Memory information:
((conses 16 195998 6534)
 (symbols 48 19735 0)
 (miscs 40 50 176)
 (strings 32 15069 5856)
 (string-bytes 1 433667)
 (vectors 16 32908)
 (vector-slots 8 653890 4234)
 (floats 8 158 12)
 (intervals 56 213 0)
 (buffers 976 18))