socket test: Adjust for Windows-specific sendto() error
socket test: Adjust for Windows-specific sendto() error

With winsock sending messages to NULL results in G_IO_ERROR_NOT_CONNECTED
instead of G_IO_ERROR_FAILED.
MSDN says:
  WSAENOTCONN
  10057
  Socket is not connected.
  A request to send or receive data was disallowed because the socket is not connected
  and (when sending on a datagram socket using sendto) no address was supplied.
So this is a direct mapping of the implementation error.
Covering it up in the wrapper (by converting it to G_IO_ERROR_FAILED)
doesn't seem feasible or needed (no one, except for the testsuite,
really cares which unrecoverable error is returned by sendto()).

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib