trx_get_trx_by_xid(): Invalidate trx->xid after a successful lookup,
so that subsequent callers will not find the same transaction.

The only callers of trx_get_trx_by_xid() will be invoking
innobase_commit_low() or innobase_rollback_trx(), and those code paths
should not depend on trx->xid.

rb://584 approved by Jimmy Yang