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/term.c b/src/term.c
index a91d2c7..99b33d4 100644
--- a/src/term.c
+++ b/src/term.c
@@ -6725,7 +6725,7 @@
     }
 
     // Check if the name is one of the colors we know
-    for (i = 0; i < (int)(sizeof(rgb_table) / sizeof(rgb_table[0])); i++)
+    for (i = 0; i < (int)ARRAY_LENGTH(rgb_table); i++)
 	if (STRICMP(name, rgb_table[i].color_name) == 0)
 	    return gui_adjust_rgb(rgb_table[i].color);