fix(proxy): remove _potential_ deadlock

The `dc.Send()` should increase the `bufferedAmount` value,
so there is no need to add the message length a second time.

Also replace GT with GE, for the case where
`BufferedAmountLowThreshold === maxBufferedAmount`

Currently the deadlock cannot happen because `maxBufferedAmount`
and `BufferedAmountLowThreshold` are too far apart, in fact
the former is 2x the latter.

See
- https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/merge_requests/144#note_2902956
- https://github.com/pion/webrtc/pull/2473
- https://github.com/pion/webrtc/pull/2474

proxy/lib/webrtcconn.go

@@ -91,7 +91,7 @@ func (c *webRTCConn) Write(b []byte) (int, error) {
 	defer c.lock.Unlock()
 	if c.dc != nil {
 		c.dc.Send(b)
-		if !c.isClosing && c.dc.BufferedAmount()+uint64(len(b)) > maxBufferedAmount {
+		if !c.isClosing && c.dc.BufferedAmount() >= maxBufferedAmount {
 			<-c.sendMoreCh
 		}
 	}