journal: fix skipping of duplicate entries in iteration

journal: fix skipping of duplicate entries in iteration

I accidentally broke the detection of duplicate entries in 7943f42275
"journal: optimize iteration by returning previously found candidate
entry".

When we have a known location of a candidate entry, we must not return
from next_beyond_location() immediately. We must go through the
duplicates detection to make sure the candidate differs from the
already iterated entry.

This fix slows down iteration a bit, but it's still faster than it
was before the rework.

$ time ./journalctl --since=2014-06-01 --until=2014-07-01 > /dev/null

real    0m4.448s
user    0m4.298s
sys     0m0.149s

(Compare with results from commit 7943f42275, where real was 5.3s before
the rework.)