patch 8.2.2920: still a way to shadow a builtin function

Problem:    Still a way to shadow a builtin function. (Yasuhiro Matsumoto)
Solution:   Check the key when using extend(). (issue #8302)
diff --git a/src/eval.c b/src/eval.c
index 9fb3b4e..7a05d35 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1462,12 +1462,8 @@
 		semsg(_(e_dictkey), lp->ll_newkey);
 		return;
 	    }
-	    if ((lp->ll_tv->vval.v_dict == get_globvar_dict()
-		    || lp->ll_tv->vval.v_dict ==
-			   &SCRIPT_ITEM(current_sctx.sc_sid)->sn_vars->sv_dict)
-		    && (rettv->v_type == VAR_FUNC
-			|| rettv->v_type == VAR_PARTIAL)
-		    && var_wrong_func_name(lp->ll_newkey, TRUE))
+	    if (dict_wrong_func_name(lp->ll_tv->vval.v_dict, rettv,
+								lp->ll_newkey))
 		return;
 
 	    // Need to add an item to the Dictionary.