Fix check for open_image() ret

When open_image() was modified to return a pointer rather than an int
in commit 295090c1, these two checks were overlooked and never fixed.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>