catalog: bound item offsets against the mmap in the binary reader

catalog: bound item offsets against the mmap in the binary reader

The binary catalog reader trusted two values straight from a (possibly
hostile) database: open_mmap() summed header_size + n_items *
catalog_item_size in uint64 with no overflow check, and find_id() added
the matched item's offset to the map base with no upper bound. Reachable
through sd_journal_get_catalog() with $SYSTEMD_CATALOG set, this let
catalog_get()/catalog_list() strdup() a string starting outside the
mapping. Guard the size math with MUL_SAFE/INC_SAFE and reject item
offsets that fall outside the file.