Exit if unable to dlopen() libminijailpreload.so.

BUG=chromium:204504
TEST=Unit tests, Minijail autotests.
TEST=Rename libminijailpreload.so, run 'minijail0 -- /usr/bin/id', see error.

Change-Id: I3978775662a7710b89f274e7ed7c265d9b923ca2
Reviewed-on: https://chromium-review.googlesource.com/182103
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org>
2 files changed