Fix: libcrmcluster: prevent external callers from triggering assertion when connecting to cluster

When sbd is connecting to cluster by calling crm_cluster_connect() ->
pcmk_cluster_connect() -> pcmkcorosync_connect() ->
pcmkcpg_connect() -> pcmk__server_message_type()

, it triggers assertion:

error: log_assertion_as: pcmkserver_message_type: Triggered fatal
assertion at servers.c:165 : (server > 0) && (server <
PCMKNELEM(server_info))

This fixes it by avoiding calling pcmkserver_message_type() in
pcmkcpg_connect().