Fix an issue where resolveKill() emits a warning if the future was never started

Summary:
Ref T2794. This is a minor warning I hit while exploring Phage stuff. In Phage, we can resolveKill() a future that we never called start() on, which tries to proc_terminate() a process which never spawned. This emits a warning.

Instead, check that $this->proc exists before signaling it.

Test Plan: Added a unit test.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2794

Differential Revision: https://secure.phabricator.com/D17354