LP#1711194: avoid division by zero errors

LP#1711194: avoid division by zero errors

Under some circumstances, osrf_control --diagnostic may not
be able to deduce the correct max_children setting for a service,
e.g., if it's run without --localhost on a system that doesn't
have a opensrf.xml config section specifying active apps for a
specific hostname. When that happens, just display the count
of running drones rather than displaying error messages about
undefined $dmax values and divisions by zero.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>