[latexila] Factory: fix return_if_fail -> return_val_if_fail

[Sébastien Wilmet <swilmet src gnome org>]

commit 7359e31c7184e51b019eea3f37e39228c7665bee
Author: Ting-Wei Lan <lantw src gnome org>
Date:   Mon Dec 25 01:50:10 2017 +0800

    Factory: fix return_if_fail -> return_val_if_fail
    
    Since the function returns a non-void value.
    
    Catched with clang.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791929

 src/factory.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/factory.vala b/src/factory.vala
index e525366..957c536 100644
--- a/src/factory.vala
+++ b/src/factory.vala
@@ -25,7 +25,7 @@ public class Factory : Tepl.AbstractFactoryVala
 
     public override Gtk.ApplicationWindow? create_main_window_vala (Gtk.Application gtk_app)
     {
-        return_if_fail (gtk_app is LatexilaApp);
+        return_val_if_fail (gtk_app is LatexilaApp, null);
         LatexilaApp app = gtk_app as LatexilaApp;
 
         MainWindow? active_main_window = app.get_active_main_window ();