api: null terminated string in cgroup_get_controller_next()

api: null terminated string in cgroup_get_controller_next()

Fix non-terminated string warnings, reported by Coverity tool:

CID 258299 (#1-2 of 2): String not null terminated (STRING_NULL).
string_null: Passing unterminated string controller.path to strcmp,
which expects a null-terminated string

This issue was reported following the path src/tools/cgsnapshot.c:
- parse_controllers()
  - cgroup_get_controller_begin()
    - cgroup_get_controller_next()

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