Freeze when count = 0 is passed

[siolag161]

When we set count=0 in add_occurrences it freezes!

Apparently the issue is with python-dateutils, like for example,

list(rrule.rrule(rrule.DAILY, dtstart=datetime(2010,1,1,1), count=0))

will likely cause the problem ( generate an infinite list of occurrences)

What do you think we should do?

[dakrauth]

I've added a simple tweak to check for false-y values for count and until in the rrule params