fix buffer overrun
fix buffer overrun

Summary: (Note: this ignores all push blocking failures!)

Reviewed By: andriigrynenko

Differential Revision: D23005233

fbshipit-source-id: 4ea528505e06fec0af7841ccbd6d41eabe0de558

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