Negative retval is error, while 0 is OK.

v2: More places

travis-ci: success for criu: Fix open() retval analysing
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>