windows: use 0 for convert flags
windows: use 0 for convert flags

CP_UTF8 must use 0.

https://msdn.microsoft.com/en-us/library/windows/desktop/dd319072%28v=vs.85%29.aspx

> Note For UTF-8 or code page 54936 (GB18030, starting with Windows
> Vista), dwFlags must be set to either 0 or
> MB_ERR_INVALID_CHARS. Otherwise, the function fails with
> ERROR_INVALID_FLAGS.

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