bug#22072: Error in display-startup-screen if create-image fails

[Richard Copley <rcopley@gmail.com>]

There's a mistake in the function "use-fancy-splash-screens-p" in
startup.el. It can set the local variable "image-height" to nil (if
creating an image from the fancy-splash-image-file fails), but later
it uses the value of "image-height" in an arithmetic expression, and
gives the error "Wrong type argument: number-or-marker-p, nil".

(The reason create-image has suddenly started failing for me is a
difference in my build environment. I'm trying out a build using only
stuff available from the MSYS2 package manager. I haven't noticed
any other problems so far. The create-image problem goes away if
I configure --without-imagemagick.)

In GNU Emacs 25.1.50.1 (x86_64-w64-mingw32)
 of 2015-12-02
Repository revision: 098738ff1a193ff6e6af71071461f75635d1b790
Windowing system distributor 'Microsoft Corp.', version 10.0.10586
Configured using:
 'configure --prefix /c/emacs/emacs-20151201-205551
 --disable-dependency-tracking
 --enable-locallisppath=%emacs_dir%/../site-lisp 'CFLAGS=-Og -g -ggdb''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND DBUS NOTIFY ACL GNUTLS
LIBXML2 ZLIB TOOLKIT_SCROLL_BARS

Important settings:
  value of $LANG: ENG
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
display-startup-screen: Wrong type argument: number-or-marker-p, nil

Features:
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mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util help-fns help-mode easymenu cl-loaddefs pcase cl-lib mail-prsvr
mail-utils advice time-date mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel dos-w32 ls-lisp disp-table
w32-win w32-vars term/common-win tool-bar dnd fontset image regexp-opt
fringe tabulated-list newcomment elisp-mode lisp-mode prog-mode register
page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
font-lock syntax facemenu font-core frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev obarray minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote w32notify dbusbind w32 multi-tty
make-network-process emacs)

Memory information:
((conses 16 84676 9280)
 (symbols 56 19141 0)
 (miscs 48 36 128)
 (strings 32 14569 3411)
 (string-bytes 1 399450)
 (vectors 16 11586)
 (vector-slots 8 411206 1940)
 (floats 8 141 49)
 (intervals 56 243 0)
 (buffers 976 11))