Race condition in xmpp subscription code causes assertion

Found by inspection when investigating a core dump.

If vrf_membership_request_map_ is non-empty when XmppChannel
close is started, but the routingtable_membership_request_map_ is
empty, the close of the XmppPeer is not deferred. If a routing
instance that's in the vrf_membership_request_map_ subsequently
gets created before the XmppPeer close process is complete, one
or more RTarget routes for the routing instance are published and
registration for the tables in the instance is triggered. These
routes cause an assertion to fail when the XmppPeer is destroyed.