fix postgres for non-pool users

lib/dialects/postgres/connector-manager.js

@@ -145,6 +145,7 @@ module.exports = (function() {
       // Closes a client correctly even if we have backed up queries
       // https://github.com/brianc/node-postgres/pull/346
       this.client.on('drain', this.client.end.bind(this.client))
+      this.client = null
     }
 
     this.isConnecting = false