libcgroup: Fix segmentation fault in cgroup_init() -take2

libcgroup: Fix segmentation fault in cgroup_init() -take2

An artifact from the time when libcgroup could not handle multiple
cgroup filesystems mounted at multiple points. It is very much possible
a subsystem is not mounted at that point. (FWIW, it is wrong even on a
single mount point, but I am quite surprised that no one hit it yet.)

V1->V2
A minor cleanup possible.

Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
git-svn-id: https://libcg.svn.sourceforge.net/svnroot/libcg/trunk@120 4f4bb910-9a46-0410-90c8-c897d4f1cd53