fix leak in list_active_containers()

fix leak in list_active_containers()

Found by running the lxc-test-list test with valgrind. The names were
put into a local array, and never freed in the success case where the
caller didn't want the names returned and in the early out failure case.

Note we don't need to check the return from remove_from_array() because
we just successfully added the name above.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>