recv_sys_t::apply(): Remove a bogus debug assertion that had been
added in commit f2c17cc9 (MDEV-29911).

It is perfectly normal that when the server was killed in the middle of
writing multiple redo log blocks, the recovery would end such that
recv_sys.scanned_lsn will point to the end of the last complete 512-byte
log block, but recv_sys.recovered_lsn will be less than that.

Also, correct the function comment of recv_sys_t::parse().