Removed unchecked access to an optional in checker process.
Removed unchecked access to an optional in checker process.

This could lead to an excutor crash when the id was set to
`None` between the check and the time it was accessed.

Review: https://reviews.apache.org/r/67345/

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