patch 9.1.1356: Vim9: crash when unletting variable

Problem:  Vim9: crash when unletting variable
Solution: fix crash, allow to use :unlet
          (Hirohito Higashi)

closes: #17226

Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim9cmds.c b/src/vim9cmds.c
index 6295090..d4ffb93 100644
--- a/src/vim9cmds.c
+++ b/src/vim9cmds.c
@@ -99,7 +99,7 @@
     if (name[1] != ':' || vim_strchr((char_u *)"gwtb", *name) == NULL)
     {
 	// "unlet s:var" is allowed in legacy script.
-	if (*name == 's' && !script_is_vim9())
+	if (*name == 's' && !in_vim9script())
 	    return OK;
 	semsg(_(e_cannot_unlet_str), name);
 	return FAIL;