Ensure active requests not decremented to negative
Ensure active requests not decremented to negative

Summary: Since the max request comparison casts active requests counter from signed to unsigned, if the active requests become negative, their unsigned value is very large and would trigger overload protection. This diff makes sure that we won't decrement the counter below zero.

Reviewed By: ycking21

Differential Revision: D38459476

fbshipit-source-id: dd9bb43fc9c1bd575d90242b73af2d3ec6d3326a

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift