Do not attempt to release undefined connections from pool, #414

If a connection is not able to be initially established, there is no need to release it from the pool.  This is currently resulting in pushing an undefined connection to release into the generic-pool-redux.

I propose checking to make the connection is not undefined before attempting to release it.  This will avoid crashing an app with the undefined method .end() error.

This is aimed to fix issue #414.