Don't try to call 'kill' on null procs.

When iOS log capture has been unsuccessful, `this.proc` never becomes
not null.  In `stopCapture`, we then attempt to kill null, which crashes Appium.

This isn't nice.
Merge pull request #3549 from DylanLacey/no_killing_null

Don't try to call 'kill' on null procs.
Enabled Performance Logs for iOS