tools/cgxget: fix segfault in free path

tools/cgxget: fix segfault in free path

cgxget tool crashes, when passing basic operations such as multiple
cgroups as arguments or invalid parameters, it turns out the address of
struct cgroup passed for cgroup_free() has been invalid.

$ sudo ./src/tools/cgxget -1 -r cpu.shares a b
Segmentation fault (core dumped)

$ sudo ./src/tools/cgxget -1 cpu.shares a b
Segmentation fault (core dumped)

this patch fixes it by passing the right address of the struct cgroup[]
to cgroup_free().

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>