Fix PERCENTILE_DISC behaviour

SELECT
  percentile_disc(0.0) WITHIN GROUP (ORDER BY a) as p0,
  percentile_disc(0.1) WITHIN GROUP (ORDER BY a) as p1,
  percentile_disc(0.2) WITHIN GROUP (ORDER BY a) as p2,
  percentile_disc(0.3) WITHIN GROUP (ORDER BY a) as p3,
  percentile_disc(0.4) WITHIN GROUP (ORDER BY a) as p4,
  percentile_disc(0.5) WITHIN GROUP (ORDER BY a) as p5,
  percentile_disc(0.6) WITHIN GROUP (ORDER BY a) as p6,
  percentile_disc(0.7) WITHIN GROUP (ORDER BY a) as p7,
  percentile_disc(0.8) WITHIN GROUP (ORDER BY a) as p8,
  percentile_disc(0.9) WITHIN GROUP (ORDER BY a) as p9,
  percentile_disc(1.0) WITHIN GROUP (ORDER BY a) as p10
FROM VALUES (0), (1), (2), (3), (4) AS v(a)

+---+---+---+---+---+---+---+---+---+---+---+
| p0| p1| p2| p3| p4| p5| p6| p7| p8| p9|p10|
+---+---+---+---+---+---+---+---+---+---+---+
|0.0|0.0|0.0|1.0|1.0|2.0|2.0|2.0|3.0|3.0|4.0|
+---+---+---+---+---+---+---+---+---+---+---+