Fix bogus size calculation introduced by commit cc5f81366.

Fix bogus size calculation introduced by commit cc5f81366.

The elements of RecordCacheArray are TupleDesc, not TupleDesc *.
Those are actually the same size, so that this error is harmless,
but it's still wrong --- and it might bite us someday, if TupleDesc
ever became a struct, say.

Per Coverity.