no QUIT in Fplist_get

[Kalle Olavi Niemitalo]

In GNU Emacs 20.6.4 (i386-debian-linux-gnu, X toolkit)
 of Thu Jun  1 2000 on PC486
configured using `configure  i386-debian-linux-gnu --prefix=/usr 
--sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var/lib 
--infodir=/usr/share/info --with-pop=yes --with-x=yes --with-x-toolkit=yes'
started using `emacs --no-init-file --no-site-file'

Evaluating the following expression puts Emacs in a loop
from which C-g does not return to top level:

(let ((cycle (list 'name 'value)))
  (nconc cycle cycle)
  (plist-get cycle 'nil))