vim-patch:9.1.0764: [security]: use-after-free when closing a buffer (#30705)

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

51b62387be

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/buffer.c

@@ -478,6 +478,11 @@ static bool can_unload_buffer(buf_T *buf)
   return can_unload;
 }
 
+bool buf_locked(buf_T *buf)
+{
+  return buf->b_locked || buf->b_locked_split;
+}
+
 /// Close the link to a buffer.
 ///
 /// @param win    If not NULL, set b_last_cursor.