When the buffer that nvim_buf_set_lines() is changing is not in any vim
window, fix_cursor() leads to calling ml_get_buf() with an invalid line
number. The condition that fix_cursor() was called on was (buf ==
curbuf), but this is always true because of the call to
switch_to_win_for_buf() earlier in the function.
Instead this should be predicated on (save_curbuf.br_buf == NULL)