bug#24727: 25.1; Editing custom theme removes undefined variables and faces

[Allen Li]

When using `customize-create-theme' to visit and edit an existing theme,
undefined variables and faces get removed on save (for example, for a
package whose autoload hasn't been triggered yet).

In GNU Emacs 25.1.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.20.9)
 of 2016-09-18 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe
 -fstack-protector-strong' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11