Fix "BYMONTHDAY" occurrences

Hello guys!

It's sad that the main repository of `ri_cal` is abandoned. But your fork state is the most updated. I use it in one project.
I got bug when tried to get occurrences for that rrule `"FREQ=YEARLY;BYMONTH=12;BYMONTHDAY=25;COUNT=10"`. This PR fixes it.

Thanks!