diagnostics_updater does not support diagnostic_period < 1

[orzechow]

I'm not sure about the python version, but the diagnostics_updater in c++ crashes with

std::runtime_error "Time is out of dual 32-bit range"

if the diagnostic_period parameter is < 1
The reason lies in diagnostics_updater::update_diagnostic_period() where ros::Duration overflows if period_ < old_period:
https://github.com/ros/diagnostics/blob/groovy-devel/diagnostic_updater/include/diagnostic_updater/diagnostic_updater.h#L520

[trainman419]

This looks like it's actually a bug in addition of a negative ros::Duration object to a ros::Time object. I'll see if I can come up with a test case to reproduce it.

[trainman419]

It turns out that this was a bug in the initialization of next_time_, resulting in it being negative if diagnostic_period was set less than the default.

Fixed in 3c7f6d4