Fixed a potential unsafe access to Container struct.
Fixed a potential unsafe access to Container struct.

This patch adds guard to the Container struct in docker
containerizer.

Review: https://reviews.apache.org/r/67585/

abudnik/mesos,andschwa/mesos,chhsia0/mesos,jpeach/mesos,gsantovena/mesos,reneploetz/mesos,gsantovena/mesos,dforsyth/mesos,kaysoky/mesos,abudnik/mesos,dforsyth/mesos,jpeach/mesos,andschwa/mesos,andschwa/mesos,reneploetz/mesos,jpeach/mesos,kaysoky/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,dforsyth/mesos,andschwa/mesos,chhsia0/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,reneploetz/mesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,kaysoky/mesos,dforsyth/mesos,Gilbert88/mesos,gsantovena/mesos,reneploetz/mesos,andschwa/mesos,jpeach/mesos,kaysoky/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos,Gilbert88/mesos