protect against emtpy wmi query result sets

If an WMI query were to return an empty result set, trying to access the
first element in the result set would result in a `panic: runtime error:
index out of range` error.

add logic to explicitly check if the result set size is 0 and return an
error rather.

Fixes #240

collector/ad.go

@@ -3,6 +3,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -616,6 +618,9 @@ func (c *ADCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, er
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.AddressBookOperationsTotal,

collector/cs.go

@@ -4,6 +4,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -60,6 +62,9 @@ func (c *CSCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, er
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.LogicalProcessors,

collector/dns.go

@@ -4,6 +4,8 @@
 package collector
 
 import (
+	"errors"
+
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/log"
@@ -237,6 +239,9 @@ func (c *DNSCollector) collect(ch chan<- prometheus.Metric) (*prometheus.Desc, e
 	if err := wmi.Query(q, &dst); err != nil {
 		return nil, err
 	}
+	if len(dst) == 0 {
+		return nil, errors.New("WMI query returned empty result set")
+	}
 
 	ch <- prometheus.MustNewConstMetric(
 		c.ZoneTransferRequestsReceived,