[sledge] Fix crash when trying to warn
[sledge] Fix crash when trying to warn

Reviewed By: mbouaziz

Differential Revision: D15518828

fbshipit-source-id: 069ff4e9c

facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer