Added missing `return` statement in `Slave::statusUpdate`.
Added missing `return` statement in `Slave::statusUpdate`.

Previously, if `statusUpdate` was called for a pending task, it would
forward the status update and then continue executing `statusUpdate`,
which then checks if there is an executor that is aware of this task.
Given that a pending task is not known to any executor, it would always
handle it by forwarding status update one more time. This patch adds
missing `return` statement, which fixes the issue.

Review: https://reviews.apache.org/r/71361

Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,Gilbert88/mesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,kaysoky/mesos,kaysoky/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,dforsyth/mesos