Real interface index can't be zero.
getsockopt(..., IP_UNICAST_IF, ...) may return success and zero
interface index if IP_UNICAST_IF was never set.

Avoids error output which doesn't inidcate a real error condition
for such case.
Signed-off-by: Paul Gofman <pgofman@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>