bug#22442: 25.0.50; --with-cairo does not display PNG on GNU/Linux

[Paul Eggert]

To reproduce the problem, configure Emacs --with-cairo, and then run these shell 
commands:

wget https://upload.wikimedia.org/wikipedia/commons/f/f5/All_palaeotemps.png
emacs -Q All_palaeotemps.png

Emacs shows a blank (or sometimes garbled) screen instead of the image.

I think the problem is that lookup_rgb_color is returning garbage.

Here are more details about the configuration, built on Fedora 23 x86-64:

In GNU Emacs 25.0.50.55 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.6, cairo 
version 1.14.2)
 of 2016-01-22 built on penguin.cs.ucla.edu
Repository revision: 27a0919cde7a6dce93aeba63b5ce7d0834f48531
Configured using:
 'configure --enable-gcc-warnings --with-cairo'

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

Important settings:
  value of $LC_ALL: en_US.utf8
  value of $LC_COLLATE: en_US.UTF-8
  value of $LC_CTYPE: en_US.UTF-8
  value of $LC_MESSAGES: en_US.UTF-8
  value of $LANG: C
  locale-coding-system: utf-8-unix