free up existing session in setSSLSession
free up existing session in setSSLSession

Summary: Free up the session if its not null before assigning a new one

Reviewed By: shamdor-fb

Differential Revision: D7169809

fbshipit-source-id: fd026e8688525a764b9937c7f4b7bfb0cdece9eb

facebook/folly,rklabs/folly,facebook/folly,rklabs/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,rklabs/folly,facebook/folly