add error return

Author: paulyufan2

cni/network/multitenancy.go

@@ -162,9 +162,12 @@ func (m *Multitenancy) DetermineSnatFeatureOnHost(snatFile, nmAgentSupportedApis
 // addDefaultRouteToGateway appends a default route
 // to both epInfo and result. Returns error if gwStr is not a valid IP.
 func (m *Multitenancy) addDefaultRouteToGateway(gwStr string, epInfo *network.EndpointInfo, result *network.InterfaceInfo) error {
-	var dst net.IPNet
-
 	gw := net.ParseIP(gwStr)
+	if gw == nil {
+		return fmt.Errorf("invalid gateway IP: %s", gwStr) //nolint:errcheck
+	}
+
+	var dst net.IPNet
 	if gw.To4() != nil {
 		_, defaultIPNet, _ := net.ParseCIDR("0.0.0.0/0")
 		dst = net.IPNet{IP: net.IPv4zero, Mask: defaultIPNet.Mask}