Eliminate a vla in header reading
Eliminate a vla in header reading

Summary: MSVC doesn't support VLA's.

Reviewed By: yfeldblum

Differential Revision: D5865051

fbshipit-source-id: 09e4ff56893a0bfe2abb701e0b9c60a8a4b42848

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift