app/vmselect/promql: return the correct time series from quantile

Previously arbitrary time series could be returned from `quantile`
depending on sort order for the last data point in the selected range.

Fix this by returning the calculated time series.

Fixes https://github.com/VictoriaMetrics/VictoriaMetrics/issues/55

app/vmselect/promql/aggr.go

@@ -457,6 +457,7 @@ func newAggrQuantileFunc(phis []float64) func(tss []*timeseries) []*timeseries {
 			idx := int(math.Round(float64(len(tss)-1) * phi))
 			dst.Values[n] = tss[idx].Values[n]
 		}
+		tss[0] = dst
 		return tss[:1]
 	}
 }