sockets: Plug memory leak in socket_address_flatten()

socket_address_flatten() leaks a SocketAddress when its argument is
null.  Happens when opening a ChardevBackend of type 'udp' that is
configured without a local address.  Screwed up in commit bd269ebc due
to last minute semantic conflict resolution.  Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1494866344-11013-1-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
