Make failover in progress check more aggressively.

Make failover in progress check more aggressively.

In pool_virtual_master_db_node_id() the case when session context is
not available was not covered by the failover in progress checking
because I thought it'd be too aggressive. However a report from field
showed that that could happen while authenticating a client (and it
causes a segfault). So I decided to move the check to beginning of the
function to cover the case.