Fix signed/unsigned mismatch in json.cpp
Fix signed/unsigned mismatch in json.cpp

Summary: Fix signed/unsigned mismatch in json.cpp

Reviewed By: yfeldblum, jdonald

Differential Revision: D19671088

fbshipit-source-id: 4bc1f4111e0536aa22c7fd03a2a6a6d786b7cfc8

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly