win32: handle ERROR_MORE_DATA

If a named pipe is being read in message mode and the next message is
longer than the nNumberOfBytesToRead parameter specifies, ReadFile
returns FALSE and GetLastError returns ERROR_MORE_DATA.

Since the API doesn't allow to return both a GError and the number of
bytes read so far, it makes more sense to return nread, and let the
client call GetLastError() himself to check if ERROR_MORE_DATA.

The current alternative loses the nread information.

https://bugzilla.gnome.org/show_bug.cgi?id=679288