session: connection pool may be nil on close
session: connection pool may be nil on close

When the connConfig errors the connection pool will not be created,
check that it is not nil before closing it.

mailgun/gocql,revinate/gocql,gocql/gocql,znly/gocql,i/gocql,nikandfor/gocql,znly/gocql,aai/gocql,ChannelMeter/gocql,CrowdSurge/gocql,gocql/gocql,gus/gocql,revinate/gocql,Zariel/gocql,vrischmann/gocql,nikandfor/gocql,gus/gocql,aai/gocql,mailgun/gocql,nikandfor/gocql,vrischmann/gocql,CrowdSurge/gocql,i/gocql,Zariel/gocql,ChannelMeter/gocql