[WS] Fix wslay multi-frame message parsing (again)

We incorrectly assumed that the `payload_length` in the recv start
callback of wslay was the final message size, but according to the
WebSocket  protocol, the payload length always refers to the current
frame's payload size.

The protocol, in fact, do not include a "message payload" length on
purpose to allow sending messages of unknown size without forcing the
sender to buffer the whole message (RFC6455 Section 5.4).

This means a receiving peer has no way to know beforehand how long a
message will be, and needs instead to keep track of the length of each
frame until the FIN one is received to properly reconstruct the message
at the end.

modules/websocket/wsl_peer.cpp

@@ -598,7 +598,6 @@ void WSLPeer::_wsl_recv_start_callback(wslay_event_context_ptr ctx, const struct
 		// Get ready to process a data package.
 		PendingMessage &pm = peer->pending_message;
 		pm.opcode = op;
-		pm.payload_size = arg->payload_length;
 	}
 }
 
@@ -608,6 +607,7 @@ void WSLPeer::_wsl_frame_recv_chunk_callback(wslay_event_context_ptr ctx, const
 	if (pm.opcode != 0) {
 		// Only write the payload.
 		peer->in_buffer.write_packet(arg->data, arg->data_length, nullptr);
+		pm.payload_size += arg->data_length;
 	}
 }