trx_undo_mem_create_at_db_start(): Do not read TRX_UNDO_TRX_NO
unless the field is known to be valid, that is, the transaction
has been serialized and trx_purge_add_undo_to_history() has been
invoked.

Normally InnoDB pages would be zero-initialized on allocation
(since MySQL 5.5 or so), but the undo log pages skip that
mechanism. So, reused undo log pages can contain garbage.
Undo log headers can start at any offset (there can be
multiple undo log headers in the same undo log page).
Therefore, because the TRX_UNDO_TRX_NO is never explicitly
initialized on undo log header creation, its contents may
be garbage.