fix(upnp): panic during a shutdown process

[ackintosh]

Description

upnp panics here in case where the gateway state is Searching and the task executor is shutting down, since the search_gateway task is terminated and the sender channel has been dropped.

Here is an example code that reproduces the issue based on the upnp example, along with the corresponding error message.

Since this case is expected during shutdown, I believe upnp should avoid panicking and continue its operation.

Notes & open questions

Should we add new variant to GatewayState and Event for this?

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[jxs]

Thanks for this Akihito! LGTM! ❤️

Approvals have been dismissed because the PR was updated after the send-it label was applied.