dont call ctdb_fatal() just because we are asked to restart a connection

dont call ctdb_fatal() just because we are asked to restart a connection
to a remote node and ctdb->methods is NULL.

This can happen when we are in the middle of a normal shutdown of the
daemon and we have already shut down the transport layer (thus setting
ctdb->methods == NULL in the transport layer destructor)
band there is some unprocessed data related to a remote node.

This prevents an ugly race condition where ctdb might sometimes (rare)
cause a core dump during "ctdb shutdown".