patch 9.1.1361: [security]: possible use-after-free when closing a buffer

patch 9.1.1361: [security]: possible use-after-free when closing a buffer

Problem:  [security]: Possible to open more windows into a closing
          buffer without splitting, bypassing existing "b_locked_split"
          checks and triggering use-after-free
Solution: Disallow switching to a closing buffer. Editing a closing
          buffer (via ":edit", etc.) was fixed in v9.1.0764, but add an
          error message and check just "b_locked_split", as "b_locked"
          is necessary only when the buffer shouldn't be wiped, and may
          be set for buffers that are in-use but not actually closing.
          (Sean Dewar)

closes: #17246

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>