engine: don't panic if TTL <= 0
engine: don't panic if TTL <= 0

Fix #933

webwlsong/fleet,WPMedia/fleet,dalbani/fleet,mookjp/fleet,octoblu/fleet,endocode/fleet,bcwaldon/fleet,bviswana101/fleet,thepwagner/fleet,dongsupark/fleet,jonboulle/fleet,mwitkow/fleet,paveltiunov/fleet,bcwaldon/fleet,dalbani/fleet,webwlsong/fleet,WPMedia/fleet,improbable-io/fleet,mwitkow/fleet,paveltiunov/fleet,ecnahc515/fleet,flandr/fleet,ecnahc515/fleet,hhoover/fleet,wuqixuan/fleet,mischief/fleet,improbable-io/fleet,dongsupark/fleet,coreos/fleet,jonboulle/fleet,mwitkow-io/fleet,octoblu/fleet,giantswarm/fleet,dalbani/fleet,mwhooker/fleet,mwitkow-io/fleet,esatterwhite/fleet,mookjp/fleet,coreos/fleet,giantswarm/fleet,mwhooker/fleet,ecnahc515/fleet,giantswarm/fleet,flandr/fleet,bcwaldon/fleet,bviswana101/fleet,mischief/fleet,esatterwhite/fleet,wuqixuan/fleet,hhoover/fleet,endocode/fleet,endocode/fleet,thepwagner/fleet,improbable-io/fleet,dongsupark/fleet,coreos/fleet,jonboulle/fleet