[sledge] Strengthen Sh re null cannot be allocated
[sledge] Strengthen Sh re null cannot be allocated

Summary:
Strengthen the Sh.seg constructor and Sh.is_false test to account for
the axiom that `null -[_;_)-> ⟨_,_⟩` is inconsistent.

Reviewed By: ngorogiannis

Differential Revision: D14481986

fbshipit-source-id: 7016e7451

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