Deal with ValueError exceptions from the email.utils timestamp code.

Got an error like this:
  File "./maildir-deduplicate.py", line 253, in get_canonical_header_value
    utc_timestamp = email.utils.mktime_tz(parsed)
  File "/usr/lib/python2.6/email/_parseaddr.py", line 146, in mktime_tz
    t = time.mktime(data[:8] + (0,))
ValueError: year out of range

This should avoid it.