The assert was caused by an error of XA transaction that had
BINLOG 'base64_string' statement.
The statement failed because of lack of checking whether the encoded
replication event was handled by the slave applier thread.
If it's not the slave applier no error should be generated, but it was
in this case, see a test added.

Fixed along with the idea borrowed the upstream to introduce a check
of which applier executes the replication event and do not
report any error if the applier is a regular server client.