bug#37786: 26.3; Emacs crashes when calling function to decode string

[Allen Li]

The following expression, when evaluated in emacs -Q, causes emacs to
crash and dump core:

(mail-decode-encoded-word-string 
#("=?UTF-8?B?4pyoUERYQ09OIFNBTEXinKggRW5qb3kgU3BlY2lhbCBPZmZlcnMg?= 
=?UTF-8?B?V2hpbGUgWW91IFdhaXQh?=" 0 64 (ws-butler-chg chg) 64 65 (ws-butler-chg 
chg) 65 97 (ws-butler-chg chg)))

In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.10)
 of 2019-08-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12005000
System Description:     Arch Linux

Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS GLIB
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD LCMS2

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