Detect end-of-stream in TransportImpl#init

OpenSSH will drop connections based on the value of MaxStartups when
there are too many unauthenticated connection. When this happens, reads
on the client socket return -1, which was previously inserted into the
identification buffer, leading to the error in hierynomus#118.

src/main/java/net/schmizz/sshj/transport/TransportImpl.java

@@ -136,7 +136,10 @@ public void init(String remoteHost, int remotePort, InputStream in, OutputStream
             // Read server's ID
             final Buffer.PlainBuffer buf = new Buffer.PlainBuffer();
             while ((serverID = readIdentification(buf)).isEmpty()) {
-                buf.putByte((byte) connInfo.in.read());
+                int b = connInfo.in.read();
+                if (b == -1)
+                    throw new TransportException("Server closed connection during identification exchange");
+                buf.putByte((byte) b);
             }
 
             log.info("Server identity string: {}", serverID);