patch 8.2.2922: computing array length is done in various ways

Problem:    Computing array length is done in various ways.
Solution:   Use ARRAY_LENGTH everywhere. (Ken Takata, closes #8305)
diff --git a/src/gui_athena.c b/src/gui_athena.c
index 4384740..f4aafcd 100644
--- a/src/gui_athena.c
+++ b/src/gui_athena.c
@@ -469,7 +469,7 @@
     if (menu->icon_builtin || gui_find_bitmap(menu->name, buf, "xpm") == FAIL)
     {
 	if (menu->iconidx >= 0 && menu->iconidx
-	      < (int)(sizeof(built_in_pixmaps) / sizeof(built_in_pixmaps[0])))
+	      < (int)ARRAY_LENGTH(built_in_pixmaps))
 	    xpm = built_in_pixmaps[menu->iconidx];
 	else
 	    xpm = tb_blank_xpm;