patch 8.2.2421: double free when using autocommand with "argdel"

Problem:    Double free when using autocommand with "argdel". (Houyunsong)
Solution:   Add the arglist_locked flag.
diff --git a/src/version.c b/src/version.c
index 3d0613c..9709e32 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2421,
+/**/
     2420,
 /**/
     2419,