net: don't throw on immediately destroyed socket

Fixes regression introduced in af249fa8.

With connect being deferred to the next tick, Socket.destroy could be
called before connect. Socket.destroy sets _connecting to false which
would cause an assertion error.

Fixes: https://github.com/nodejs/io.js/issues/2250
PR-URL: https://github.com/nodejs/io.js/pull/2251


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>