If getenv returns a null pointer we get a segfault here.

Let's exit in a more sane way.

travis-ci: success for series starting with [1/2] zdtm:mnt_ext_auto: avoid segfault if ZDTM_NEWNS is not set
Signed-off-by: Dmitrii Shcherbakov <dshcherbakov@virtuozzo.com>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>