bug#29116: 24.5; Parsing dates from VCAL

[Vegard Vesterheim]

I received an email containing a VCAL attachment which Gnus is unable to
render, the error in the minibuffer says:

  Wrong type argument: char-or-string-p, nil

The VCAL-attachment contains this:

    DTSTART;VALUE=DATE:20171208

The spec for DTSTART states that the value type for DTSTART can be
either DATE-TIME or DATE, so this seems like a valid format, so I am
inclined to think that this is a bug in emacs.

Here is the backtrace (parameters removed 
  timezone-make-arpa-date(2000 0 0 "01:00:00" (3600 "CET"))
  timezone-make-date-arpa-standard("20171208" nil nil)
  gnus-icalendar-event--decode-datefield(...)
  gnus-icalendar-event-from-ical(...)
  gnus-icalendar-event-from-buffer(...)
  gnus-icalendar-event-from-handle(...)
  gnus-icalendar-mm-inline(...)
  mm-display-inline(...)
  mm-display-part(...)
  gnus-mime-display-alternative(...)
  gnus-mime-display-part(..)
  gnus-display-mime()
  gnus-article-prepare-display()
  gnus-article-prepare(94599 nil)
  gnus-summary-display-article(94599)
  gnus-summary-next-article(nil)

Looking at the backtrace it seems that the problem is related to parsing
the date string: "20171208".