Because InnoDB is not freeing undo pages to the normal
free-page management, old undo log pages can be reused. Due to that,
it is possible (but unlikely) that the fields TRX_UNDO_NEEDS_PURGE
and TRX_UNDO_LOG_START relative to the free offset that is stored at
TRX_UNDO_PAGE_HDR + TRX_UNDO_PAGE_START already have the correct value.
Hence, we must pass the mtr_t::OPT template parameter to silence the
debug assertion.
Other writes in trx_undo_header_create() that are using the default
template parameter seem to be correct (the data fields should be
guaranteed to change even in the event of reusing pages).
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