patch 9.1.0678: [security]: use-after-free in alist_add()

Problem:  [security]: use-after-free in alist_add()
          (SuyueGuo)
Solution: Lock the current window, so that the reference to
          the argument list remains valid.

This fixes CVE-2024-43374

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/structs.h b/src/structs.h
index fe4704a..abda3a0 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3785,7 +3785,7 @@
     synblock_T	*w_s;		    // for :ownsyntax
 #endif
 
-    int		w_closing;	    // window is being closed, don't let
+    int		w_locked;	    // window is being closed, don't let
 				    // autocommands close it too.
 
     frame_T	*w_frame;	    // frame containing this window