The fix of MDEV-13654 (commit ff81faf6)
wrongly caused ADD PRIMARY KEY to ignore duplicate PRIMARY KEY values
caused by concurrent DML transactions that had been started before the
ALTER TABLE operation (but did not access the table before the ALTER TABLE
started).

row_ins_duplicate_online(): Always report a duplicate key error
if DB_TRX_ID had been reset (it belongs to a transaction that had
started before the ALTER TABLE operation).