Clear request in doAppOverloadedException()
Clear request in doAppOverloadedException()

Summary: Calling `doAppOverloadedException()` currently logs an error in the callback destructor.

Differential Revision: D21708223

fbshipit-source-id: f9feb6ffa290bd324b5d7194e02d61a23b2eabca

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