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/map.c b/src/map.c
index 2e792fe..9923522 100644
--- a/src/map.c
+++ b/src/map.c
@@ -2478,13 +2478,12 @@
     if (!gui.starting)
 #  endif
     {
-	for (i = 0;
-		i < (int)(sizeof(cinitmappings) / sizeof(struct initmap)); ++i)
+	for (i = 0; i < (int)ARRAY_LENGTH(cinitmappings); ++i)
 	    add_map(cinitmappings[i].arg, cinitmappings[i].mode);
     }
 # endif
 # if defined(FEAT_GUI_MSWIN) || defined(MACOS_X)
-    for (i = 0; i < (int)(sizeof(initmappings) / sizeof(struct initmap)); ++i)
+    for (i = 0; i < (int)ARRAY_LENGTH(initmappings); ++i)
 	add_map(initmappings[i].arg, initmappings[i].mode);
 # endif
 #endif