User can cause infinite loop with recurrence widget

[laulaz]

If you set an interval ("repeat every") value of "0" in reccurrence popup, it causes an infinite loop in Python dateutil RRULE interpretation.

Example of a simple RRULE that breaks : FREQ=MONTHLY;INTERVAL=0;BYMONTHDAY=1;COUNT=7'

INTERVAL=0 is not a valid value for RFC 5545 and should not be allowed.

I can see in the translations that the error message exists :

plone.formwidget.recurrence/plone/formwidget/recurrence/browser/i18n.py

Line 150 in e95c341

noRepeatEvery=translate(_("no_repeat_every"), context=request),

Also, in the Mockup demo, there is an error message for "0" interval :

But in the last Plone 6, you can stil choose "0" interval and put the instance on her knees with infinite loop.