Offline / Back Office Renewal fatal error when "renewal_date" set to far back.

While testing CRM-15861, I ran into this issue.

Using latest civibuild, when renewing & backdating the "renewal_date" too far, it is possible to get a scary looking screen that looks like the attached schreenshot.

Steps to reproduce.

Using 4.7.20, find any membership.  Click renew.

Enter a "renewal date" that is before the "member since date".

Submit.

Expected Results:

Not sure, but most reasonably, you get an error saying "renewal date cannot be before members since date or before current membership start date".

Actual Results:

You get the deadly error above.