fix nullptr-with-nonzero-offset in folly/io/IOBuf.cpp
fix nullptr-with-nonzero-offset in folly/io/IOBuf.cpp

Reviewed By: yfeldblum

Differential Revision: D30232471

fbshipit-source-id: 1137e306fca34f687c99c7f2155a03515926da45

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