Check ssl_ member for null before invoking SSL_pending
Check ssl_ member for null before invoking SSL_pending

Summary:
`checkForImmediateRead` is invoked when you set the read callback.
This can happen prior to setting `ssl_`, so let's just check that it's
non-null before handing it off.

Test Plan: Tested it within larger mcrouter diff

Reviewed By: alandau@fb.com

FB internal diff: D1253826


uber/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,maxwell-demon/fbthrift,kangkot/fbthrift,maxwell-demon/fbthrift,LinusU/fbthrift,raphaelamorim/fbthrift,LinusU/fbthrift,kangkot/fbthrift,guker/fbthrift,chjp2046/fbthrift,HankFaan/fbthrift,maxwell-demon/fbthrift,maxwell-demon/fbthrift,jesboat/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,maxwell-demon/fbthrift,getyourguide/fbthrift,facebook/fbthrift,uber/fbthrift,SergeyMakarenko/fbthrift,Orvid/fbthrift,HankFaan/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,fengshao0907/fbthrift,HankFaan/fbthrift,facebook/fbthrift,getyourguide/fbthrift,uber/fbthrift,project-zerus/fbthrift,maxwell-demon/fbthrift,sinjar666/fbthrift,kangkot/fbthrift,SergeyMakarenko/fbthrift,HankFaan/fbthrift,soumith/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,soumith/fbthrift,SergeyMakarenko/fbthrift,uber/fbthrift,guker/fbthrift,sinjar666/fbthrift,Orvid/fbthrift,sinjar666/fbthrift,raphaelamorim/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,maxwell-demon/fbthrift,guker/fbthrift,chjp2046/fbthrift,LinusU/fbthrift,fengshao0907/fbthrift,raphaelamorim/fbthrift,maxwell-demon/fbthrift,project-zerus/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,soumith/fbthrift,sinjar666/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,jesboat/fbthrift,project-zerus/fbthrift,kangkot/fbthrift,guker/fbthrift,fengshao0907/fbthrift,uber/fbthrift,jesboat/fbthrift,chjp2046/fbthrift,raphaelamorim/fbthrift,fengshao0907/fbthrift,sinjar666/fbthrift,facebook/fbthrift,Orvid/fbthrift,jesboat/fbthrift,HankFaan/fbthrift,project-zerus/fbthrift,maxwell-demon/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,SergeyMakarenko/fbthrift,raphaelamorim/fbthrift,uber/fbthrift,kangkot/fbthrift,HankFaan/fbthrift,raphaelamorim/fbthrift,raphaelamorim/fbthrift,HankFaan/fbthrift,LinusU/fbthrift,soumith/fbthrift,facebook/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,project-zerus/fbthrift,raphaelamorim/fbthrift,guker/fbthrift,sinjar666/fbthrift,getyourguide/fbthrift,chjp2046/fbthrift,project-zerus/fbthrift,HankFaan/fbthrift,jesboat/fbthrift,jesboat/fbthrift,kangkot/fbthrift,uber/fbthrift,soumith/fbthrift,soumith/fbthrift,fengshao0907/fbthrift,LinusU/fbthrift,guker/fbthrift,jesboat/fbthrift,fengshao0907/fbthrift,guker/fbthrift,uber/fbthrift,guker/fbthrift,sinjar666/fbthrift,jesboat/fbthrift,chjp2046/fbthrift,soumith/fbthrift,kangkot/fbthrift,fengshao0907/fbthrift,facebook/fbthrift,facebook/fbthrift,soumith/fbthrift,uber/fbthrift,guker/fbthrift,jesboat/fbthrift,uber/fbthrift,fengshao0907/fbthrift,sinjar666/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,kangkot/fbthrift,getyourguide/fbthrift,fengshao0907/fbthrift,kangkot/fbthrift,chjp2046/fbthrift,Orvid/fbthrift,sinjar666/fbthrift,maxwell-demon/fbthrift,kangkot/fbthrift,soumith/fbthrift,SergeyMakarenko/fbthrift,project-zerus/fbthrift,project-zerus/fbthrift,uber/fbthrift,HankFaan/fbthrift,raphaelamorim/fbthrift,soumith/fbthrift,HankFaan/fbthrift,chjp2046/fbthrift,jesboat/fbthrift,Orvid/fbthrift,LinusU/fbthrift,maxwell-demon/fbthrift,Orvid/fbthrift,project-zerus/fbthrift,guker/fbthrift,chjp2046/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,kangkot/fbthrift,raphaelamorim/fbthrift,Orvid/fbthrift,uber/fbthrift,project-zerus/fbthrift,fengshao0907/fbthrift,sinjar666/fbthrift,HankFaan/fbthrift,guker/fbthrift,sinjar666/fbthrift,fengshao0907/fbthrift,raphaelamorim/fbthrift,LinusU/fbthrift,chjp2046/fbthrift,project-zerus/fbthrift,jesboat/fbthrift,Orvid/fbthrift,facebook/fbthrift,soumith/fbthrift,jesboat/fbthrift,project-zerus/fbthrift