Add Missing Binlog Log Event Validation

A binlog event that is malformed or deliberately crafted can make the
server or mariadb-binlog read, and in some cases write, outside the
event buffer while the event is being parsed. The outcome depends on
the event type and the field. Reports so far include a crash of the
slave SQL thread that takes the whole server down, a crash of
mariadb-binlog, a replica that applies wrong data, and sanitizer
reports of heap buffer overflows.

Such an event reaches the parser from (1) a master the attacker
controls, (2) an unencrypted replication stream the attacker can
modify, (3) a BINLOG statement run by a user with the
BINLOG REPLAY privilege, and (4) a binlog or relay log file given to
mariadb-binlog. The event checksum does not stop this, since whoever
writes the event also computes its checksum.

Each event type is parsed by its own Log_event subclass
constructor, called from Log_event::read_log_event(). The
constructor takes lengths, counts and offsets from the event bytes
themselves, and uses them to size allocations, advance the read pointer
and copy data. The slave IO and SQL threads, the BINLOG statement
and mariadb-binlog all parse events through these constructors.

The constructors check some of these values against the end of the
event and not others, and some existing checks cannot catch a large
value. Each external report so far has found one more such field, and
each was filed and fixed on its own ticket: MDEV-39485, MDEV-39660,
MDEV-39689, MDEV-39762, MDEV-40365, MDEV-40366, MDEV-40643, MDEV-40644,
MDEV-40645, MDEV-40647, MDEV-40648. A fix for one field does not cover
the next one, and a bound added for one report can itself be bypassed.
For example, the column count bound in Table_map_log_event added by
MDEV-39689 adds the count to the read pointer and compares the sum to
the end of the event. A column count sent as the single byte 0xfb
decodes to NULL_LENGTH, the sum wraps, the bound passes, and the
following memcpy() runs with a length of SIZE_MAX.

This ticket aims to encompass future missing validation to do a pass
over the log event code to add many missing validations.