[analyzer] DanglingDelegate: better null pointer checks

Summary:
[analyzer] DanglingDelegate: better null pointer checks

Based on crash report P18481427.

Test Plan: `make test`

Reviewers: hansonw, adamjernst

Reviewed By: adamjernst

Subscribers: hannesr

Differential Revision: https://phabricator.fb.com/D1738794

Tasks: 5814394

Signature: t1:1738794:1418663057:927cbb95da3112717a0113dc1b85513028eb451f