gtk_tree_selection_real_modify_range() has a g_return_if_fail() if the
start or end paths passed to it do not correspond to real tree nodes.
However, GtkTreePaths inherently do not have to be valid, so it should
be acceptable to call gtk_tree_selection_select_range() with
non-existent paths. Replace the g_return_if_fail() by a silent return,
and add a unit test.
https://bugzilla.gnome.org/show_bug.cgi?id=712760
