patch 8.2.1342: Vim9: accidentally using "t" gives a confusing error

Problem:    Vim9: accidentally using "x" gives a confusing error.
Solution:   Disallow using ":t" in Vim9 script. (issue #6399)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index ad1693b..081d9e4 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -7276,6 +7276,9 @@
 {
     long	n;
 
+    if (not_in_vim9(eap) == FAIL)
+	return;
+
     n = get_address(eap, &eap->arg, eap->addr_type, FALSE, FALSE, FALSE, 1);
     if (eap->arg == NULL)	    // error detected
     {