fix(autocmd): potential TabClosed UAF, always set abuf

Problem: win_free_mem can free w_buffer (via qf_free_all), which may cause a
heap use-after-free if used as TabClosed's <abuf>. I think TabClosed is also the
only event to conditionally set <abuf> not based on event type.

Solution: use the buffer saved by the bufref. Fall back to curbuf if invalid,
like WinResized/WinScrolled.

NOTE: Not always equivalent if close_buffer autocmds switch buffers at the last
moment; previously <abuf> would be set to that buffer. Fixed in next commit.

https://github.com/neovim/neovim/actions/runs/21765657455/job/62800643599?pr=37758#step:9:159
for an example of qf_free_all being a nuisance.

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