updated for version 7.3.560
Problem:    Get an error for a locked argument in extend().
Solution:   Initialize the lock flag for a dictionary. (Yukihiro Nakadaira)
diff --git a/src/eval.c b/src/eval.c
index 83e2016..1b28200 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -19981,6 +19981,7 @@
     dictitem_T	*dict_var;
 {
     hash_init(&dict->dv_hashtab);
+    dict->dv_lock = 0;
     dict->dv_refcount = DO_NOT_FREE_CNT;
     dict->dv_copyID = 0;
     dict_var->di_tv.vval.v_dict = dict;