Handle crazy offset to SHOW BINLOG EVENTS

SHOW BINLOG EVENTS FROM <"random"-pos> caused a variety of failures as reported in MDEV-18046. They are fixed but that approach is not future-proof as well as is not optimal to create extra check for being constructed
event parameters.
The extra checks can and should be avoided with conduction validation
to the supplied offset. The validation means scan of the requested log
to find the requested position as one that points to the beginning of an event. Scanning is meaningful only to a non-default binlog-checksum-off files (that fact is captured by MDEV-18046).