psppire-output-view: Only render output items once the window is realized.

Otherwise, in environments where the output view is not rendered
immediately upon the call to gtk_widget_show_all() from
psppire_output_submit(), psppire_output_view_put() called later in that
function would dereference a null pointer trying to create a rendering
from a null GTK_WIDGET (view->output)->window.

Bug #43362.