zdtm/cgroup_ifpriomap: Fix Coverity warning

*** CID 185302:  Null pointer dereferences  (NULL_RETURNS)
/test/zdtm/static/cgroup_ifpriomap.c: 107 in read_one_priomap()
>>>     Dereferencing a pointer that might be null "out->ifname" when calling "strncpy".

There is also a warning about using rand(), but..
Not sure that we need to entangle everything just for pleasing Coverity:
>>>     CID 185301:  Security best practices violations  (DC.WEAK_CRYPTO)
>>>     "rand" should not be used for security related applications, as linear congruential algorithms are too easy to break.
Leaving that as-is and marking in Coverity as WONTFIX.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>