BUG/MEDIUM: servers: Only destroy a conn_stream we just allocated.

BUG/MEDIUM: servers: Only destroy a conn_stream we just allocated.

In connect_server(), if we failed to add the connection to the session,
only destroy the conn_stream if we just allocated it, otherwise it may
have been allocated outside connect_server(), along with a connection which
has its destination address set.
Also use si_release_endpoint() instead of cs_destroy(), to make sure the
stream_interface doesn't reference it anymore.

This should be backported to 1.9.