use a buffered channel in the acceptQueue (#53)

There's a race condition here. A user might call Next to obtain the next
stream, and get a nil result. He then calls Chan get notified about new
incoming streams. If a new stream is accepted between the call to Next and the
call to Chan, he won't be notified about that stream, unless the channel is
buffered.

session.go

@@ -22,15 +22,18 @@ const closeWebtransportSessionCapsuleType http3.CapsuleType = 0x2843
 
 type acceptQueue[T any] struct {
 	mx sync.Mutex
-	c  chan struct{}
+	// The channel is used to notify consumers (via Chan) about new incoming items.
+	// Needs to be buffered to preserve the notification if an item is enqueued
+	// between a call to Next and to Chan.
+	c chan struct{}
 	// Contains all the streams waiting to be accepted.
 	// There's no explicit limit to the length of the queue, but it is implicitly
 	// limited by the stream flow control provided by QUIC.
 	queue []T
 }
 
 func newAcceptQueue[T any]() *acceptQueue[T] {
-	return &acceptQueue[T]{c: make(chan struct{})}
+	return &acceptQueue[T]{c: make(chan struct{}, 1)}
 }
 
 func (q *acceptQueue[T]) Add(str T) {