crushtool: safeguard for missing --num-rep when --test

http://tracker.ceph.com/issues/9490 Fixes: #9490

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2508 from dachary/wip-9490-crushtool-num-rep

crushtool: safeguard for missing --num-rep when --test

Reviewed-by: Johnu George <johnugeorge109@gmail.com>