quic: handle streams moving from the data queue to the meta queue

In Conn.appendStreamFrames, a stream can be moved from the
data queue (for streams with only flow-controlled frames to send)
to the metadata queue (for streams with non-flow-controlled frames
to send) if some other goroutine asynchronously modifies the
stream state.

Adjust the check at the end of this function to clear the needSend
bool only if queueMeta and queueData are both empty, to avoid
losing track of the need to send frames when this happens.

For golang/go#58547

Change-Id: Ib9ad3b01f543cd7673f5233ceb58b2db9adfff5a
Reviewed-on: https://go-review.googlesource.com/c/net/+/531656
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

internal/quic/conn_streams.go

@@ -372,7 +372,9 @@ func (c *Conn) appendStreamFrames(w *packetWriter, pnum packetNumber, pto bool)
 		state = s.state.set(0, streamQueueData)
 		c.queueStreamForSendLocked(s, state)
 	}
-	c.streams.needSend.Store(c.streams.queueData.head != nil)
+	if c.streams.queueMeta.head == nil && c.streams.queueData.head == nil {
+		c.streams.needSend.Store(false)
+	}
 	return true
 }