patch 9.1.0764: [security]: use-after-free when closing a buffer

Problem:  [security]: use-after-free when closing a buffer
Solution: When splitting the window and editing a new buffer,
          check whether the newly to be edited buffer has been marked
          for deletion and abort in this case

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-rj48-v4mq-j4vg

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/version.c b/src/version.c
index 2f37123..c8559ef 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    764,
+/**/
     763,
 /**/
     762,