sd-journal: do not reset sd_j_enumerate_unique position on error

sd-journal: do not reset sd_j_enumerate_unique position on error

systemctl would call sd_j_enumerate_unique() interleaved with
sd_j_next(). But the latter can remove a file if it detects an
error in it. In those circumstances sd_j_enumerate_unique would
restart with the first file in hashmap. With many corrupted files
sd_j_enumerate_unique might iterate over the list multiple times.

Avoid this by jumping to the next file in unique list if possible,
or setting a flag that tells sd_j_enumerate_unique that it is done
otherwise.