patch 9.1.0115: Using freed memory with full tag stack and user data

Problem:  Using freed memory with full tag stack and user data
          (Konstantin Khlebnikov)
Solution: Clear the user data pointer of the newest entry.
          (zeertzjq, Konstantin Khlebnikov)

fixes: neovim/neovim#27498
closes: #14053

Co-authored-by: Konstantin Khlebnikov koct9i@gmail.com
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Konstantin Khlebnikov koct9i@gmail.com
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index fcd402c..356d74e 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -1,4 +1,4 @@
-*testing.txt*	For Vim version 9.1.  Last change: 2024 Jan 23
+*testing.txt*	For Vim version 9.1.  Last change: 2024 Feb 18
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar