Fix bug in DebugProtocol printing if IOBuf is null
Fix bug in DebugProtocol printing if IOBuf is null

Reviewed By: yfeldblum

Differential Revision: D25139304

fbshipit-source-id: 5889d0ac565a8e2cba6cb2b421fb60b20968fb4e

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