mingw: forbid translating ERROR_SUCCESS to an errno value

mingw: forbid translating ERROR_SUCCESS to an errno value

Johannes Sixt pointed out that the `err_win_to_posix()` function
mishandles `ERROR_SUCCESS`: it maps it to `ENOSYS`.

The only purpose of this function is to map Win32 API errors to `errno`
ones, and there is actually no equivalent to `ERROR_SUCCESS`: the idea
of `errno` is that it will only be set in case of an error, and left
alone in case of success.

Therefore, as pointed out by Junio Hamano, it is a bug to call this
function when there was not even any error to map.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>