Fix two more SIZE_T_CEILING issues

This patch imposes (very long) limits on the length of a line in a
directory document, and on the length of a certificate.  I don't
think it should actually be possible to overrun these remotely,
since we already impose a maximum size on any directory object we're
downloading, but a little defensive programming never hurt anybody.

Roger emailed me that doorss reported these on IRC, but nobody seems
to have put them on the bugtracker.