Fix undefined memory access
Fix undefined memory access

Summary: For whatever reason, the next diff in the stack brought out the issue.

Reviewed By: beicy

Differential Revision: D38416188

fbshipit-source-id: acb70f0424957dbf513f124d203353b549efcc72

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex