This regression for debug builds was introduced by
MDEV-23101 (commit 224c9504).

Due to MDEV-16664, the parameter
innodb_lock_schedule_algorithm=VATS
is not enabled by default.

The purpose of the added assertions was to enforce the invariant that
Galera replication cannot be enabled together with VATS due to MDEV-12837.
However, upon closer inspection, it is obvious that the variable 'lock'
may be assigned to the null pointer if no match is found in the
previous->hash list.

lock_grant_and_move_on_page(), lock_grant_and_move_on_rec():
Assert !lock->trx->is_wsrep() only after ensuring that lock
is not a null pointer.