nspawn: free D-Bus error before fallback (#39131)

Otherwise the next call fails:

Got message type=error sender=:1.5 destination=:1.470 path=n/a 
    interface=n/a member=n/a cookie=151 reply_cookie=3 signature=s
    error-name=org.freedesktop.DBus.Error.UnknownMethod
    error-message=Unknown method CreateMachineEx or interface
    org.freedesktop.machine1.Manager. Assertion
'!bus_error_is_dirty(reterr_error)' failed at
    src/libsystemd/sd-bus/sd-bus.c:2396, function sd_bus_call(). Aborting.

Follow-up for d80af3b97b

src/nspawn/nspawn-register.c

@@ -294,6 +294,7 @@ int register_machine(
                 return 0;
         if (!sd_bus_error_has_name(&error, SD_BUS_ERROR_UNKNOWN_METHOD))
                 return log_error_errno(r, "Failed to register machine: %s", bus_error_message(&error, r));
+        sd_bus_error_free(&error);
         if (FLAGS_SET(flags, REGISTER_MACHINE_KEEP_UNIT)) {
                 r = bus_call_method(
                                 bus,