Continue to back away from the LEFT JOIN optimization of check-in [41c27bc0ff1d3135]
by disallowing query flattening if the outer query is DISTINCT.  Without this fix,
if an index scan is run on the table within the view on the right-hand side of the
LEFT JOIN, stale result registers might be accessed yielding incorrect results,
and/or an OP_IfNullRow opcode might be invoked on the un-opened table, resulting
in a NULL-pointer dereference.  This problem was found by the Yongheng and Rui fuzzer.

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