fix union deserialization bug for cpp2
fix union deserialization bug for cpp2

Summary: Missed a spot on the first try :(

Test Plan: fixed our bizarre bugs

Reviewed By: davejwatson@fb.com

FB internal diff: D1238782

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