ifpps: Don't bug out if interface went down

In case the interface ifpps is running on went down, ifpps bugged out
because the stats didn't look plausible anymore, due to the IRQ count
being reset:

ifpps: ifpps.c:563: stats_diff: Assertion `!((new->irqs[i] - old->irqs[i]) > (new->irqs[i]))' failed

Fix this by setting the difference to 0 in this case. ifpps will thus
display a irqs/t value of 0 instead of bugging out.

This fixes #74.

Reported-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>