Check the return value from DropRoot()

Unlike the other brillo::Minijail methods, this one can fail if the
username lookup does not succeed.

BUG=chromium:670985
TEST=buildbots

Change-Id: Id1d5059fb155c1a227033af0eba837ab37e96fe9
Reviewed-on: https://chromium-review.googlesource.com/416302
Commit-Ready: Kevin Cernekee <[email protected]>
Tested-by: Kevin Cernekee <[email protected]>
Reviewed-by: Mattias Nissler <[email protected]>
Reviewed-by: Jorge Lucangeli Obes <[email protected]>
1 file changed