Avoid NULL-pointer dereference. Apparently monitor.{c,h} calls
lxc_check_inherited() with NULL passed for the config. This isn't really a big
issue since monitor.{c,h} is effectively dead for all liblxc versions that have
the state client patch. Also, the patch that introduces the relevant lines into
lxc_check_inherited() is only in master and yet unreleased.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>