Without this check, if -o is not given, compel will fail with
the following error:

	Can't open (null)

It's non-trivial to figure out that it means -o is required.

While at it, initialize the corresponding field.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>