patch 8.2.1713: Motif GUI: crash when setting menu colors

Problem:    Motif GUI: crash when setting menu colors. (Andrzej Bylicki)
Solution:   Add {} to make "n" incremented correctly. (closes #6989,
            closes #5948)
diff --git a/src/gui_motif.c b/src/gui_motif.c
index 9d13b57..57d50b0 100644
--- a/src/gui_motif.c
+++ b/src/gui_motif.c
@@ -916,9 +916,13 @@
 # endif
 	{
 	    if (gui.menu_bg_pixel != INVALCOLOR)
+	    {
 		XtSetArg(arg[0], XmNbackground, gui.menu_bg_pixel); n++;
+	    }
 	    if (gui.menu_fg_pixel != INVALCOLOR)
+	    {
 		XtSetArg(arg[1], XmNforeground, gui.menu_fg_pixel); n++;
+	    }
 	    menu->submenu_id = XmCreatePopupMenu(textArea, "contextMenu",
 								      arg, n);
 	    menu->id = (Widget)0;