app/vmselect/promql: fix panic on a single zero vmrange bucket in prometheus_buckets() function

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/296

app/vmselect/promql/exec_test.go

@@ -2964,6 +2964,23 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r1, r2, r3, r4, r5, r6}
 		f(q, resultExpected)
 	})
+	t.Run(`prometheus_buckets(zero-vmrange-value)`, func(t *testing.T) {
+		t.Parallel()
+		q := `sort(prometheus_buckets(label_set(0, "vmrange", "0...0")))`
+		r1 := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{0, 0, 0, 0, 0, 0},
+			Timestamps: timestampsExpected,
+		}
+		r1.MetricName.Tags = []storage.Tag{
+			{
+				Key:   []byte("le"),
+				Value: []byte("+Inf"),
+			},
+		}
+		resultsExpected := []netstorage.Result{r1}
+		f(q, resultsExpected)
+	})
 	t.Run(`prometheus_buckets(valid)`, func(t *testing.T) {
 		t.Parallel()
 		q := `sort(prometheus_buckets((

app/vmselect/promql/transform.go

@@ -363,6 +363,7 @@ func vmrangeBucketsToLE(tss []*timeseries) []*timeseries {
 			ts := xs.ts
 			if isZeroTS(ts) {
 				// Skip time series with zeros. They are substituted by xssNew below.
+				xsPrev = xs
 				continue
 			}
 			if xs.start != xsPrev.end {