Fix segfault on lxc-create when no template specified

When no template file is specified on lxc-create, recieve segfault.
So change not to append header in config when no template is specified.

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>