Be more robust about nil pointers in protobuf

While not strictly correct, it can easily happen that proto messages
are created that use nil pointers instead of pointers in empty strings
to denote an empty string.

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/registry.go

@@ -727,8 +727,8 @@ func checkMetricConsistency(
 	}
 
 	for _, labelPair := range dtoMetric.GetLabel() {
-		if !utf8.ValidString(*labelPair.Value) {
-			return fmt.Errorf("collected metric's label %s is not utf8: %#v", *labelPair.Name, *labelPair.Value)
+		if !utf8.ValidString(labelPair.GetValue()) {
+			return fmt.Errorf("collected metric's label %s is not utf8: %#v", labelPair.GetName(), labelPair.GetValue())
 		}
 	}