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/terminal.c b/src/terminal.c
index 2189d01..e3945d0 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -5691,7 +5691,7 @@
 
     if (attr > HL_ALL)
 	attr = syn_attr2attr(attr);
-    for (i = 0; i < sizeof(attrs)/sizeof(attrs[0]); ++i)
+    for (i = 0; i < ARRAY_LENGTH(attrs); ++i)
 	if (STRCMP(name, attrs[i].name) == 0)
 	{
 	    rettv->vval.v_number = (attr & attrs[i].attr) != 0 ? 1 : 0;