Don't attempt to process packets when there are none

Under certain unlikely circumstances the main server loop could attempt to process packets even when the connection didn't return one. This would result in the default empty packet being processed resulting in spurious warnings about a missing client.

src/server.cpp

@@ -1076,6 +1076,8 @@ void Server::Receive(float timeout)
 				// and a faster server-step is better than busy waiting.
 				if (remaining_time_us() < 1000.0f)
 					break;
+				else
+					continue;
 			}
 
 			peer_id = pkt.getPeerId();