Fix for xembedsniproxy crash due to NULL returned from xcb_image_get()

Summary:
In my environment (Fedora 27, plasma-workspace-5.11.4-1.fc27.x86_64) xembedsniproxy crashes.
This is caused by NULL being returned by xcb_image_get().

This should fix or at least mitigate the problems from bugs:
https://bugs.kde.org/show_bug.cgi?id=355463
https://bugs.kde.org/show_bug.cgi?id=359664
https://bugzilla.redhat.com/show_bug.cgi?id=1497829

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D9732