When the result of a subquery is to be stored in a register and that

When the result of a subquery is to be stored in a register and that
subquery has an ORDER BY clause and an OFFSET, NULL out the destination
register before starting the ORDER BY so that the register will be set
correctly even if the OFFSET is larger than the number of output rows.
Fix for the problem reported in
[forum:/forumpost/0ec80f12d02acb3f|forum post 0ec80f12d02acb3f].

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