If trx_free() and trx_create_low() were called while a call to
trx_reference() was pending, we could get a reference to a wrong
transaction object.

trx_reference(): Return NULL if the trx->id no longer matches.

lock_trx_release_locks(): Assign trx->id = 0, so that trx_reference()
will not return a reference to this object.

trx_cleanup_at_db_startup(): Assign trx->id = 0.

assert_trx_is_free(): Assert !trx->n_ref. Assert trx->id == 0,
now that it will be cleared as part of a transaction commit.