tests: fix a memory leak in cgpath

tests: fix a memory leak in cgpath

```
==131188==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 568 byte(s) in 1 object(s) allocated from:
    #0 0x7f8918943bc8 in malloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10dbc8)
    #1 0x7f89181ee5a3 in lxc_container_new /home/vagrant/lxc/src/lxc/lxccontainer.c:5264
    #2 0x55ffc5022869 in test_container /home/vagrant/lxc/src/tests/cgpath.c:176
    #3 0x55ffc5023023 in main /home/vagrant/lxc/src/tests/cgpath.c:233
    #4 0x7f891709e0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>