Defensive coding: guard against a division by zero

Defensive coding: guard against a division by zero

In the function print_per_system_stats(), if packets have been
lost, the number of sent packets is checked to be positive before
dividing by it.  If no packets have been lost, this is not checked.
Either the existing check is not needed, or both code paths need
the check.

The function print_per_system_splits() is quite similar to
print_per_system_stats(), and has the equivalent guards against a
division by zero in both code paths, not just one of them.

In the spirit of defensive coding, I think it is better to be safe
and add the missing guard against a division by zero.