fix(nvim_open_win): crash if autocmds delete buffer/window #15549

win_set_buf can trigger autocmds if noautocmd=false. If they close the window,
code afterwards will dereference the freed win_T* wp pointer.

This interaction became possible after commit 1def3d1542d6a65f057e743faea39a760b50db87.

The reason deleting curbuf crashes, and not the buf passed to
`nvim_open_win`, is because the float initially edits curbuf (`win_init`)
until it's later set to edit buf (windows from `:new` and `:split <buf>`
behave similiarly: approx. `:split`, then `:buffer <buf>`).

`do_buffer` closes windows when their edited buffer is deleted (unless
it's the only window; N/A for floats), so the float closes when curbuf
is deleted, so we need to check `win_valid` after `win_set_buf` too.

Closes #15548

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