RFR: 8192154 - JVM crashes inside some chroot environments on linux

[Bob Vandette]

Please review this robustness fix to the Container support recently added to Hotspot.

BUG:
https://bugs.openjdk.java.net/browse/JDK-8192154

WEBREV:
http://cr.openjdk.java.net/~bobv/8192154/webrev/

In the event that there is a partial setup of the cgroup subsystems, it’s
possible that the VM will crash.

The change moves NULL checking earlier in the scanning of the cgroup subsystems.

Tested with jtreg container tests and Mach5.

Bob.