windows: fix buffer overflow issue of uv__getnameinfo_work()

According to MSDN page
(http://msdn.microsoft.com/en-us/library/windows/desktop/ms738531.aspx),
"NodeBufferSize" and "ServiceBufferSize" parameters of GetNameInfoW()
function are expected to fill the number of WCHAR, not the buffer size
of memory layout.

Assume the value of NI_MAXHOST is 1025, and "sizeof(host)" would get 2050.
A buffer overflow issue will be triggered by it. Fix it with ARRAY_SIZE()
macro.