webrtc: Panics for non-stun packets

[lexnv]

Discovered during testing a polkadot node with litep2p + webrtc and integrating with smol-dot/smoldot#1806.

Substrate PR used for enabling webrtc: paritytech/polkadot-sdk#4310

Repro case: port scanning

sudo nmap -p 33999 -sU my-ip

litep2p/src/transport/webrtc/mod.rs

Lines 387 to 396 in 34f6435

	msg => {
	if let Err(error) = self.opening.get_mut(&source).expect("to exist").on_input(msg) {
	tracing::error!(
	target: LOG_TARGET,
	?error,
	?source,
	"failed to handle inbound datagram"
	);
	}
	}

2024-05-07 16:03:55.886 DEBUG tokio-runtime-worker litep2p::webrtc: received non-stun message source=192.168.0.8:33669

====================

Version: 1.10.0-1c04ef63751

   0: sp_panic_handler::set::{{closure}}
   1: std::panicking::rust_panic_with_hook
   2: std::panicking::begin_panic_handler::{{closure}}
   3: std::sys_common::backtrace::__rust_end_short_backtrace
   4: rust_begin_unwind
   5: core::panicking::panic_fmt
   6: core::option::expect_failed
   7: <litep2p::transport::webrtc::WebRtcTransport as futures_core::stream::Stream>::poll_next
   8: <litep2p::transport::manager::TransportContext as futures_core::stream::Stream>::poll_next
   9: litep2p::transport::manager::TransportManager::next::{{closure}}
  10: <sc_network::litep2p::Litep2pNetworkBackend as sc_network::service::traits::NetworkBackend<B,H>>::run::{{closure}}
  11: sc_service::build_network_future::{{closure}}::{{closure}}::{{closure}}
  12: <futures_util::future::poll_fn::PollFn<F> as core::future::future::Future>::poll
  13: <sc_service::task_manager::prometheus_future::PrometheusFuture<T> as core::future::future::Future>::poll
  14: <futures_util::future::select::Select<A,B> as core::future::future::Future>::poll
  15: <tracing_futures::Instrumented<T> as core::future::future::Future>::poll
  16: tokio::runtime::park::CachedParkThread::block_on
  17: tokio::runtime::context::runtime::enter_runtime
  18: <tokio::runtime::blocking::task::BlockingTask<T> as core::future::future::Future>::poll
  19: tokio::runtime::task::core::Core<T,S>::poll
  20: tokio::runtime::task::harness::Harness<T,S>::poll
  21: tokio::runtime::blocking::pool::Inner::run
  22: std::sys_common::backtrace::__rust_begin_short_backtrace
  23: core::ops::function::FnOnce::call_once{{vtable.shim}}
  24: std::sys::pal::unix::thread::Thread::new::thread_start
  25: start_thread
             at ./nptl/pthread_create.c:442:8
  26: __GI___clone3
             at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81


Thread 'tokio-runtime-worker' panicked at 'to exist', /home/lexnv/.cargo/git/checkouts/litep2p-2515ad90543f141a/eaec7ce/src/transport/webrtc/mod.rs:360

This is a bug. Please report it at:

	https://github.com/paritytech/polkadot-sdk/issues/new