patch 8.2.2318: Vim9: string and list index work differently

Problem:    Vim9: string and list index work differently.
Solution:   Make string index work like list index. (closes #7643)
diff --git a/src/list.c b/src/list.c
index 56ee5a5..4531e58 100644
--- a/src/list.c
+++ b/src/list.c
@@ -924,7 +924,7 @@
 	if (!range)
 	{
 	    if (verbose)
-		semsg(_(e_listidx), n1);
+		semsg(_(e_listidx), n1_arg);
 	    return FAIL;
 	}
 	n1 = n1 < 0 ? 0 : len;