[RFC] fix #1027: :wundo segfault in new, non-empty buffer

@justinmk before my commit `os_getperm` was only called on windows, on unix `mch_stat` was used. (https://github.com/neovim/neovim/commit/edbb687b73877d8db8b1c6e376e3422bb85a11a9#diff-c834cc70a58ebdb9c3740e3fca1a7b2fL1139)
`os_get_file_info` which gets called on the next line replaces `mch_stat`.
As `os_getperm` and `os_get_file_info` are now ported to use libuv they are both implemented with `uv_fs_stat`, so they should definitely fail for the same reasons.
If `os_getperm` failed the `os_get_file_info` should fail too.