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/misc2.c b/src/misc2.c
index 08e6ed9..0553c2c 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -1050,7 +1050,6 @@
     if (entered_free_all_mem)
 	return;
     entered_free_all_mem = TRUE;
-
     // Don't want to trigger autocommands from here on.
     block_autocmds();
 
@@ -2542,7 +2541,7 @@
     // NOTE: When adding a long name update MAX_KEY_NAME_LEN.
 };
 
-#define KEY_NAMES_TABLE_LEN (sizeof(key_names_table) / sizeof(struct key_name_entry))
+#define KEY_NAMES_TABLE_LEN ARRAY_LENGTH(key_names_table)
 
 /*
  * Return the modifier mask bit (MOD_MASK_*) which corresponds to the given