A GTID event can have variable length, with contributing factors
such as the variable length from the flags2 and optional extra flags
fields. These fields are bitmaps, where each set bit indicates an
additional value that should be appended to the event, e.g.
multi-engine transactions append a number to indicate the number of
additional engines a transaction uses. However, if a flags bit is
set, and no additional fields are appended to the event, MDEV-33672
reports that the server can still try to read from memory as if it
did exist. Note, however, in debug builds, this condition is
asserted for FL_EXTRA_MULTI_ENGINE.

This patch fixes this to check that the length of the event is
aligned with the expectation set by the flags for FL_PREPARED_XA,
FL_COMPLETED_XA, and FL_EXTRA_MULTI_ENGINE.

Reviewed By
============
Kristian Nielsen <knielsen@knielsen-hq.org>