server: allow calling Close() without Serve()
server: allow calling Close() without Serve()

Useful for CI tests in other projects where a BeforeEach()
might create the server for multiple tests, but one or more
tests doesn't actually use it and thus doesn't call Serve().

Signed-off-by: Dan Williams <aeade43d0f8ae14e7c44fa81fe17c1635ae376fe@redhat.com>

socketplane/libovsdb