go-winio: Prevent Data Race when accessing closing
go-winio: Prevent Data Race when accessing closing

The race usually happens when `closeHandle()` and `prepareIo()` are called
concurrently; the former tries to set `closing` to `true` the latter tries
to read its value.

In order to avoid this issue, we added a lock around the variable.

Signed-off-by: Kenfe-Mickael Laventure <10e26728841e39109c2809b6029d4b0763bd81d3@gmail.com>

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