tkutil.c: check type

* ext/tk/tkutil/tkutil.c (cbsubst_get_subst_key): ensure the
  result type of a method call to be an array before accessing by
  RARRAY macros.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50656 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/tk/tkutil/tkutil.c

@@ -1461,6 +1461,7 @@ cbsubst_get_subst_key(self, str)
     char *buf, *ptr, *key;
 
     list = rb_funcall(cTclTkLib, ID_split_tklist, 1, str);
+    Check_Type(list, T_ARRAY);
     len = RARRAY_LEN(list);
 
     inf = cbsubst_get_ptr(self);