gtktiledviewer: don't crash on opening non-existing files

(cherry picked from commit 43b2103f)

Conflicts:
	libreofficekit/qa/gtktiledviewer/gtktiledviewer.cxx
	libreofficekit/source/gtk/lokdocview.cxx

Change-Id: Ic48adaf038e8fbcc86a94b5e351d2f963fcfcd16