patch 9.1.0130: [security]: UAF if win_split_ins autocommands delete "wp"

Problem:  heap-use-after-free in win_splitmove if Enter/Leave
          autocommands from win_split_ins immediately closes "wp".
Solution: check that "wp" is valid after win_split_ins.
          (Sean Dewar)

closes: #14078

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/window.c b/src/window.c
index 66a1d7d..7123780 100644
--- a/src/window.c
+++ b/src/window.c
@@ -1954,7 +1954,8 @@
     }
 
     // If splitting horizontally, try to preserve height.
-    if (size == 0 && !(flags & WSP_VERT))
+    // Note that win_split_ins autocommands may have immediately closed "wp"!
+    if (size == 0 && !(flags & WSP_VERT) && win_valid(wp))
     {
 	win_setheight_win(height, wp);
 	if (p_ea)