patch 8.2.4024: confusing error message if imported name is used directly

Problem:    Confusing error message if imported name is used directly.
Solution:   Give a better error message.
diff --git a/src/userfunc.c b/src/userfunc.c
index f9bf5e3..7446d6e 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -5170,7 +5170,7 @@
 	    dbg_check_breakpoint(eap);
 
 	// Handle a function returning a Funcref, Dictionary or List.
-	if (handle_subscript(&arg, &rettv,
+	if (handle_subscript(&arg, NULL, &rettv,
 			   eap->skip ? NULL : &EVALARG_EVALUATE, TRUE) == FAIL)
 	{
 	    failed = TRUE;