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

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>