[sledge] Fix assert when Fpath.v passed empty string
[sledge] Fix assert when Fpath.v passed empty string

Summary:
It is possible for the filename of a source location to be the empty
string. Fpath.v asserts when passed an empty string.

Reviewed By: jvillard

Differential Revision: D27188304

fbshipit-source-id: a7d73444b

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