When rolling back an incomplete transaction in purge, tolerate missing
BLOBs also in update undo, when undoing an INSERT by updating a delete-marked
record, and the delete-marked record is no longer needed.
Previously, we only tolerated missing BLOBs in insert undo.
This merely fixes a debug assertion; the code performed correctly
without UNIV_DEBUG.

rb://249 approved by Sunny Bains.