patch 8.1.0989: various small code ugliness

Problem:    Various small code ugliness.
Solution:   Remove pointless NULL checks. Fix function calls. Fix typos.
            (Dominique Pelle, closes #4060)
diff --git a/src/gui_gtk_f.c b/src/gui_gtk_f.c
index d81474b..6146fe6 100644
--- a/src/gui_gtk_f.c
+++ b/src/gui_gtk_f.c
@@ -561,7 +561,7 @@
 	     * gtk_widget_size_allocate() in advance with a well-posed
 	     * allocation for a given child widget in order to set a
 	     * certain private GtkWidget variable, called
-	     * widget->priv->alloc_need, to the proper value; othewise,
+	     * widget->priv->alloc_need, to the proper value; otherwise,
 	     * gtk_widget_draw() fails and the relevant scrollbar won't
 	     * appear on the screen.
 	     *