Fix: missing nil check for error condition

Server/main.go

@@ -112,8 +112,10 @@ func main() {
 		if generateServerNetworkInterface != "" {
 			var err error
 			serverIPaddress, err = psiphon.GetInterfaceIPAddress(generateServerNetworkInterface)
-			fmt.Printf("generate failed: %s\n", err)
-			os.Exit(1)
+			if err != nil {
+				fmt.Printf("generate failed: %s\n", err)
+				os.Exit(1)
+			}
 		}
 
 		tunnelProtocolPorts := make(map[string]int)