dsdb sort test: avoid exception with fewer elements

dsdb sort test: avoid exception with fewer elements

The FIENDISH_TESTS list has 33 members, and when the number of
elements being tested is less than that (because you run the script
with, say, --elements=20) some will have a zero count. A recent fix
for Python 2.6 didn't take these possible zeros into account.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>