Fix an issue where resolveKill() emits a warning if the future was never started
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.
Reviewed By: chad
Maniphest Tasks: T2794
Differential Revision: https://secure.phabricator.com/D17354