Fix plpgsql to avoid reference to already-freed memory when returning a

pass-by-reference data type and the RETURN statement is within an EXCEPTION
block.  Bug introduced by my fix of 2007-01-28 to use per-subtransaction
ExprContexts/EStates; since that wasn't back-patched into older branches,
only 8.2 and HEAD are affected.  Per report from Gary Winslow.