journalctl: fix assertion failure in ellipsize_mem()

journalctl: fix assertion failure in ellipsize_mem()

When showing the journal through "journalctl --no-pager", if the
prefix of the log message (i.e. the date and syslog identifier) is
less than 3 characters shorter than the width of the terminal, you
get:

Assertion 'new_length >= 3' failed at src/shared/util.c:3859, function ellipsize_mem(). Aborting.

because there is not enough space for the "...".  This patch add the
necessary check.