On empty buffers, when editing the first line, the line is buffered, causing offset to be < 0. While the buffer is not actually empty, the buffered line has not been flushed (and should not be) yet, so the call is valid but an edge case.
This caused segfaults when calling win_findbuf in an `on_detach`
callback, when the callback was triggered when closing the last window
containing the buffer.
Sloppy code inherited from Vim caused user scripts to be able
to observe the cursor line in an invalid intermediary state,
due to Neovim change callbacks being unbuffered unlike Vim listeners.
Manifested in Vimscript executed from the callback possibly erroring
when `:call`:ing any function,
due to the implicit range `curwin->w_cursor.lnum,curwin->w_cursor.lnum`
failing validation.
Fixed by deferring the call to `changed_lines()` until after
`curwin->w_cursor.lnum` gets its correct value.
