vim-patch:9.1.2085: Use-after-free in winframe_remove()

Problem:  Use-after-free in winframe_remove() (henices)
Solution: Set window_layout_locked() inside winframe_remove()
          and check that writing diff files is disallowed when the
          window layout is locked.

It can happen with a custom diff expression when removing a window:

 1. Buffer was removed, so win_frame_remove() is called to remove the
    window.
 2. win_frame_remove() → frame_new_height() → scroll_to_fraction()
    → diff_check_fill() (checks for filler lines)
 3. diff_check_fill() ends up causing a diff_try_update, and because we
    are not using internal diff, it has to first write the file to a
    buffer using buf_write()
 4. buf_write() is called for a buffer that is not contained within a
    window, so it first calls aucmd_prepbuf() to create a new temporary
    window before writing the buffer and then later calls
    aucmd_restbuf(), which restores the previous window layout, calling
    winframe_remove() again, which will free the window/frame structure,
    eventually freeing stuff that will still be accessed at step 2.

closes: vim/vim#19064

https://github.com/vim/vim/commit/ead1dda74a485ef0470e7252d07c1a36b8cde517

Nvim doesn't have this bug as Nvim uses a floating window as autocommand
window, and removing it doesn't need winframe_remove().

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

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