fix iis collector panic

If the `dst_cache` slice is empty, the program panics when trying to access `dst_cache[0]`

added a `if len(dst_cache) > 0 {}` block around the use of that variable to prevent that panic.

collector/iis.go

@@ -1738,6 +1738,7 @@ func (c *IISCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 		return nil, err
 	}
 
+	if len(dst_cache) > 0 {
 		ch <- prometheus.MustNewConstMetric(
 			c.ServiceCache_ActiveFlushedEntries,
 			prometheus.GaugeValue,
@@ -1946,6 +1947,7 @@ func (c *IISCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 			prometheus.CounterValue,
 			float64(dst_cache[0].OutputCacheTotalFlushes),
 		)
+	}
 
 	return nil, nil
 }