Fix building with C++14, which errors out due to bool -> T* conversions
See also: https://bugs.gentoo.org/show_bug.cgi?id=593994

--- a/src/dialogs.cc
+++ b/src/dialogs.cc
@@ -255,7 +255,7 @@
 	const Gtk::TreePath& tpath, bool)
 {
 	Gtk::TreeIter iter = store->get_iter (tpath);
-	return iter ? iter->get_value (cols.property) : false;
+	return iter ? iter->get_value (cols.property) : NULL;
 }