[gdk-pixbuf] tests: Add test for CVE-2017-6312

[Bastien Nocera <hadess src gnome org>]

commit a6303ad765882555cf1b278a09be5f9e4cf3a39d
Author: Bastien Nocera <hadess hadess net>
Date:   Thu Nov 30 02:39:19 2017 +0100

    tests: Add test for CVE-2017-6312
    
    Provided by Ariel Zelivansky <ariel zelivans gmail com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=779012

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diff --git a/tests/test-images/fail/bug779012.ico b/tests/test-images/fail/bug779012.ico
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index 0000000..8f67185
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