daemon.ContainerExport(): do not panic
daemon.ContainerExport(): do not panic

In case ContainerExport() is called for an unmounted container, it leads
to a daemon panic as container.BaseFS, which is dereferenced here, is
nil.

To fix, do not rely on container.BaseFS; use the one returned from
rwlayer.Mount().

Fixes: 7a7357dae1bccc ("LCOW: Implemented support for docker cp + build")
Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

KarthikNayak/docker,jameszhan/docker,maxamillion/docker,johnstep/moby-moby,jvanz/docker,ibuildthecloud/docker,tophj-ibm/moby,jwhonce/docker,rremer/moby,allencloud/docker,scotttlin/docker,bearstech/moby,SvenDowideit/docker,nalind/docker,allencloud/docker,YuPengZTE/docker,flavio/docker,Mokolea/docker,allencloud/docker,cpuguy83/docker,tmp6154/docker,yosifkit/docker,djs55/docker,trebonian/docker,paralin/docker,nawawi/docker,kolyshkin/moby,nalind/docker,rremer/moby,dims/docker,crosbymichael/docker,x1022as/docker,tonistiigi/docker,scotttlin/docker,nawawi/docker,rancher/docker,crosbymichael/docker,moul/docker,jvanz/docker,tmp6154/docker,tophj-ibm/moby,unclejack/docker,Ramzec/docker,jlhawn/docker,trebonian/docker,dims/docker,djs55/docker,jvanz/docker,moby/moby,johnstep/moby-moby,flavio/docker,konstruktoid/docker-upstream,duglin/docker,mikebrow/docker,cpuguy83/docker,yosifkit/docker,vdemeester/docker,johnstep/moby-moby,bearstech/moby,konstruktoid/docker-upstream,thaJeztah/docker,maxamillion/docker,thaJeztah/docker,jimjag/docker,kolyshkin/moby,laijs/moby,vdemeester/docker,konstruktoid/docker-upstream,mikebrow/docker,YuPengZTE/docker,AkihiroSuda/docker,nalind/docker,x1022as/docker,nawawi/docker,justincormack/docker,laijs/moby,nalind/docker,konstruktoid/docker-upstream,vdemeester/docker,kolyshkin/docker,docker/docker,dmcgowan/docker,ibuildthecloud/docker,unclejack/docker,paralin/docker,moby/moby,jthelin/docker,tophj-ibm/docker,KarthikNayak/docker,laijs/moby,paralin/docker,bearstech/moby,Ramzec/docker,kojiromike/docker,yosifkit/docker,thaJeztah/docker,mahak/docker,konstruktoid/docker-1,pradipd/moby,trebonian/docker,bearstech/moby,docker/docker,moul/docker,jlhawn/docker,scotttlin/docker,estesp/docker,Ramzec/docker,nawawi/docker,docker/docker,Ramzec/docker,jameszhan/docker,crosbymichael/docker,ibuildthecloud/docker,moul/docker,gnailuy/docker,konstruktoid/docker-1,jimjag/docker,yongtang/docker,jimjag/docker,kolyshkin/docker,tophj-ibm/docker,rancher/docker,dmcgowan/docker,tophj-ibm/docker,pradipd/moby,laijs/docker,justincormack/docker,moul/docker,unclejack/docker,Mokolea/docker,flavio/docker,jwhonce/docker,cpuguy83/docker,YuPengZTE/docker,paralin/docker,duglin/docker,tophj-ibm/moby,tonistiigi/docker,jthelin/docker,Mokolea/docker,cpuguy83/docker,estesp/docker,gnailuy/docker,duglin/docker,kolyshkin/moby,thaJeztah/docker,moby/moby,x1022as/docker,tonistiigi/docker,jwhonce/docker,mahak/docker,trebonian/docker,SvenDowideit/docker,kojiromike/docker,pradipd/moby,ibuildthecloud/docker,rancher/docker,jameszhan/docker,mahak/docker,moby/moby,mahak/docker,mikebrow/docker,AkihiroSuda/docker,konstruktoid/docker-1,dmcgowan/docker,yongtang/docker,flavio/docker,Mokolea/docker,SvenDowideit/docker,estesp/docker,AkihiroSuda/docker,laijs/docker,dims/docker,kojiromike/docker,KarthikNayak/docker,maxamillion/docker,rremer/moby,ibuildthecloud/docker,tmp6154/docker,djs55/docker,gnailuy/docker,yongtang/docker,AkihiroSuda/docker,gnailuy/docker,laijs/docker,jlhawn/docker,justincormack/docker,jimjag/docker,yosifkit/docker