Fix unchecked type assertion in Subscription Stats

Fixes the following panic:
github.com/gopcua/opcua.(*Subscription).StatsWithContext(0xc0000a48c0, {0xf66c28, 0x1545c20})
            github.com/gopcua/opcua@v0.3.11/subscription.go:315 +0x207
 github.com/gopcua/opcua.(*Subscription).Stats(...)
            github.com/gopcua/opcua@v0.3.11/subscription.go:296

subscription.go

@@ -276,8 +276,15 @@ func (s *Subscription) Stats(ctx context.Context) (*ua.SubscriptionDiagnosticsDa
 		return nil, errors.Errorf("empty SubscriptionDiagnostics for sub=%d", s.SubscriptionID)
 	}
 
-	for _, eo := range v.Value().([]*ua.ExtensionObject) {
-		stat := eo.Value.(*ua.SubscriptionDiagnosticsDataType)
+	extensions, ok := v.Value().([]*ua.ExtensionObject)
+	if !ok {
+		return nil, errors.Errorf("unable to find SubscriptionDiagnostics extension objects for sub=%d", s.SubscriptionID)
+	}
+	for _, eo := range extensions {
+		stat, ok := eo.Value.(*ua.SubscriptionDiagnosticsDataType)
+		if !ok {
+			continue
+		}
 
 		if stat.SubscriptionID == s.SubscriptionID {
 			return stat, nil