buf_dblwr_t::recover(): Correct a debug assertion failure that had
been added in commit bb47e575de (MDEV-34830).
The server may have been killed while a log write was in progress, and
therefore recv_sys.scanned_lsn may be up to RECV_PARSING_BUF_SIZE bytes
ahead of recv_sys.recovered_lsn.
Thanks to Matthias Leich for providing "rr replay" traces and
testing this.