poll: Prevent false-negative from WAKE_EVENT() on W32

poll: Prevent false-negative from WAKE_EVENT() on W32

SetEvent() seems to not call SetLastError(0) internally, so checking last
error after calling SetEvent() may return the error from an earlier W32 API
call. Fix this by calling SetlastError(0) explicitly.

Currently WAKE_EVENT() code is cramped into a macro and doesn't look to be
entirely correct. Particularly, it does not check the return value of
SetEvent(), only the thread-local W32 error value. It is likely that SetEvent()
actually just returns non-zero value, but the code mistakenly thinks that the
call has failed, because GetLastError() seems to indicate so.

https://bugzilla.gnome.org/show_bug.cgi?id=733805