[enforced-env][EZ] Do not crash when env-entry doesn't exist
[enforced-env][EZ] Do not crash when env-entry doesn't exist

Summary:
Changelog: [internal]

The env-entry at the given loc might not exist, because the entry might represent an illegal write. (e.g. assignment to a const, redeclaration)

Reviewed By: panagosg7

Differential Revision: D38328123

fbshipit-source-id: c5790fa67a7a010ecdaf3ccd43aecbd66297f68a

facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow