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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

compat/mingw.c

@@ -114,6 +114,7 @@ int err_win_to_posix(DWORD winerr)
 	case ERROR_SHARING_BUFFER_EXCEEDED: error = ENFILE; break;
 	case ERROR_SHARING_VIOLATION: error = EACCES; break;
 	case ERROR_STACK_OVERFLOW: error = ENOMEM; break;
+	case ERROR_SUCCESS: BUG("err_win_to_posix() called without an error!");
 	case ERROR_SWAPERROR: error = ENOENT; break;
 	case ERROR_TOO_MANY_MODULES: error = EMFILE; break;
 	case ERROR_TOO_MANY_OPEN_FILES: error = EMFILE; break;