daemon/checkpoint: rm extra checks
daemon/checkpoint: rm extra checks

In this code, err is already checked to be nil (or non-nil), so no need
to repeat extra checks.

Fixes the following govet warnings:

> daemon/checkpoint.go:38:12: nilness: tautological condition: nil == nil (govet)
> 		case err == nil:
> 		         ^
> daemon/checkpoint.go:45:12: nilness: tautological condition: nil == nil (govet)
> 		case err == nil && stat.IsDir():
> 		         ^
> daemon/checkpoint.go:47:12: nilness: tautological condition: nil == nil (govet)
> 		case err == nil:
> 		         ^

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

thaJeztah/docker,konstruktoid/docker-1,nalind/docker,gnailuy/docker,konstruktoid/docker-upstream,Ramzec/docker,ibuildthecloud/docker,moby/moby,Mokolea/docker,estesp/docker,docker/docker,thaJeztah/docker,yosifkit/docker,estesp/docker,ibuildthecloud/docker,trebonian/docker,estesp/docker,gnailuy/docker,dims/docker,Mokolea/docker,cpuguy83/docker,yongtang/docker,flavio/docker,kojiromike/docker,docker/docker,moul/docker,paralin/docker,gnailuy/docker,moby/moby,kojiromike/docker,SvenDowideit/docker,mahak/docker,gnailuy/docker,moby/moby,bearstech/moby,jthelin/docker,moby/moby,vdemeester/docker,cpuguy83/docker,AkihiroSuda/docker,Ramzec/docker,Mokolea/docker,moul/docker,moul/docker,yosifkit/docker,thaJeztah/docker,justincormack/docker,Ramzec/docker,SvenDowideit/docker,AkihiroSuda/docker,jameszhan/docker,ibuildthecloud/docker,konstruktoid/docker-upstream,vdemeester/docker,Mokolea/docker,AkihiroSuda/docker,ibuildthecloud/docker,mahak/docker,nalind/docker,cpuguy83/docker,konstruktoid/docker-upstream,dims/docker,jimjag/docker,nawawi/docker,jvanz/docker,paralin/docker,jimjag/docker,bearstech/moby,paralin/docker,SvenDowideit/docker,konstruktoid/docker-1,trebonian/docker,konstruktoid/docker-1,docker/docker,trebonian/docker,thaJeztah/docker,konstruktoid/docker-upstream,yosifkit/docker,jameszhan/docker,jimjag/docker,AkihiroSuda/docker,nalind/docker,bearstech/moby,justincormack/docker,kojiromike/docker,mahak/docker,moul/docker,flavio/docker,yosifkit/docker,jameszhan/docker,djs55/docker,jimjag/docker,jvanz/docker,nawawi/docker,yongtang/docker,Ramzec/docker,bearstech/moby,yongtang/docker,pradipd/moby,ibuildthecloud/docker,cpuguy83/docker,dims/docker,jvanz/docker,justincormack/docker,flavio/docker,djs55/docker,mahak/docker,pradipd/moby,vdemeester/docker,nawawi/docker,nalind/docker,flavio/docker,trebonian/docker,pradipd/moby,djs55/docker,nawawi/docker,paralin/docker,jthelin/docker