bug#38549: 27.0.50; Infloop when doing completion-at-point in shell-mode

[Michael Welsh Duggan]

Hitting TAB after a semicolon in a shell buffer causes an infloop in
`shell--parse-pcomplete-arguments'.

emacs -Q
M-x shell RET
; TAB

In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, X toolkit)
 of 2019-12-09 built on pd4.ta.sei.cmu.edu
Repository revision: bb8f46d55bb369589ae475e47ad948da88c0d31d
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Red Hat Enterprise Linux Server 7.6 (Maipo)

Configured using:
 'configure --without-toolkit-scroll-bars --with-wide-int
 --prefix=/home/mwd/ --with-jpeg=ifavailable
 PKG_CONFIG_PATH=/opt/rh/devtoolset-8/root/usr/lib64/pkgconfig'

Configured features:
XAW3D XPM TIFF GIF PNG SOUND GSETTINGS GLIB NOTIFY INOTIFY LIBSELINUX
GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB LUCID X11 XDBE
XIM MODULES THREADS PDUMPER GMP

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

-- 
Michael Welsh Duggan
(mwd@cert.org)