Fix IOBuf::createCombined()
Fix IOBuf::createCombined()

Summary:
IOBuf::createCombined was derefencing a nullptr in case of allocation failure.
This patches fixes this issue by throwing std::bad_alloc if malloc() returns a
nullptr.

Reviewed By: yfeldblum

Differential Revision: D9763331

fbshipit-source-id: 7d9b9265079e74f5c63c8c8809099de97342f54d

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