Fix _endCallback

[fanatid]

If Pool#end happens between client initiate connection and receiving callback with error, _endCallback will be never called.

[charmander]

Is it possible to include a test?

[fanatid]

I tried do this, but found this is not trivial

[fanatid]

Any updates? Tests required for each PR?

[charmander]

You should be able to write the test by making a net.Server that does nothing with the client it accepts but call pool.end() and close the connection. There are a few similar tests that already exist (look for createServer).