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>