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>

src/buffer.c

@@ -497,6 +497,12 @@ can_unload_buffer(buf_T *buf)
     return can_unload;
 }
 
+    int
+buf_locked(buf_T *buf)
+{
+    return buf->b_locked || buf->b_locked_split;
+}
+
 /*
  * Close the link to a buffer.
  * "action" is used when there is no longer a window for the buffer.