Fix the reentrant_allocator loop
Fix the reentrant_allocator loop

Summary:
[Folly] Fix the `reentrant_allocator` internal loop, which when finding a new head node would incorrectly preserve the free `offset` corresponding to the previous head.

Hat tip to hpmv for the report.

Differential Revision: D23593517

fbshipit-source-id: b4fc701f08056184225e7feac32d60aabf7ea6cc

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