make sure channelClosed is never called with nullptr
make sure channelClosed is never called with nullptr

Summary: Added more logging in D1192995, but sometimes channelClosed is called with nullptr, and logging crashes.  doh

Test Plan:
fbconfig -r thrift/lib/cpp2; fbmake runtests
fbgs channelClosed(nullptr)

@override-unit-failures

Reviewed By: praveenr@fb.com

FB internal diff: D1194343

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