Calling between() on a Rrule with malformed dtstart hangs.

[ghost]

The following code will make rrule hang (enter infinite loop i guess):

    const dateISO = undefined;
    const rRule = '';
    const rruleOptions = rrule.RRule.parseString(rRule);

    rruleOptions.dtstart = new Date(dateISO);
    const rrule = new rrule.RRule(rruleOptions);

    const ruleDates = rRule.between(afterDate, beforeDate, true);

Basically the code is, if somehow you have an rrule with a malformed dtstart, between() execution will hang somewhere in the execution of _iter() as its var date value is:

{
day: NaN
hour: NaN
millisecond: 0
minute: NaN
month: NaN
second: NaN
year: NaN
} 

EDIT: noticed after submitting that there is a PR (#62) that seems to address this