bug#17558: 24.4.50; global-subword-mode breaks ERC

[Dima Kogan <dima@secretsauce.net>]

Hi. I'm using a very recent build of emacs from the VCS. With this build
ERC gets very confused if global-subword-mode is on. I believe it worked
fine just a few months ago.

Recipe:

1. emacs -Q
2. (global-subword-mode)
3. M-x erc
4. Default values for the server, nickname, password, etc are fine to
show the issue

I observe the following two error repeated many times:

error in process filter: erc-button-add-nickname-buttons: Wrong type argument: integer-or-marker-p, nil
error in process filter: Wrong type argument: integer-or-marker-p, nil

ERC kinda works even with those issues, but the data comes back from the
server sporadically and it's not really usable.

Emacs details:

In GNU Emacs 24.4.50.1 (x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2014-05-21 on lilbastard, modified by Debian
 (emacs-snapshot package, version 2:20140521-1)
Windowing system distributor `The X.Org Foundation', version 11.0.11501000
System Description:	Debian GNU/Linux testing (jessie)

Configured using:
 `configure --build x86_64-linux-gnu --host x86_64-linux-gnu
 --prefix=/usr --sharedstatedir=/var/lib --libexecdir=/usr/lib
 --localstatedir=/var --infodir=/usr/share/info/emacs-snapshot
 --mandir=/usr/share/man --with-pop=yes
 --enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/24.4.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.4.50/site-lisp:/usr/share/emacs/site-lisp
 --with-crt-dir=/usr/lib/x86_64-linux-gnu/ --with-x=yes
 --with-x-toolkit=lucid 'CFLAGS=-DDEBIAN -DSITELOAD_PURESIZE_EXTRA=5000
 -g -O2' CPPFLAGS=-D_FORTIFY_SOURCE=2 'LDFLAGS=-g -Wl,--as-needed
 -znocombreloc''

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF
GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF
XFT ZLIB

Important settings:
  value of $LC_ALL: C
  value of $LANG: C
  locale-coding-system: nil