patch 8.2.3158: strange error message when using islocked() with a number

Problem:    Strange error message when using islocked() with a number.
            (Yegappan Lakshmanan)
Solution:   Check that the name is empty.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 060b5bb..029c8a6 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5961,7 +5961,10 @@
     if (end != NULL && lv.ll_name != NULL)
     {
 	if (*end != NUL)
-	    semsg(_(e_trailing_arg), end);
+	{
+	    semsg(_(lv.ll_name == lv.ll_name_end
+					   ? e_invarg2 : e_trailing_arg), end);
+	}
 	else
 	{
 	    if (lv.ll_tv == NULL)