api.c: fix dereference after null check

api.c: fix dereference after null check

Fix dereference after null check warning, reported by Coverity tool:

CID 258308 (#1 of 1): Dereference after null check (FORWARD_NULL).
var_deref_model: Passing null pointer controller to strncmp, which
dereferences it.

Fix the warning in cgroup_get_controller_version(), by checking for the
an empty controller in the case of cgroup v1.

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