Fix other instances of attempting to close `nil` on handshake failure.

Bug introduced in e52258edac, not in any
released version of obfs4proxy.

transports/obfs2/obfs2.go

@@ -113,8 +113,9 @@ func (cf *obfs2ClientFactory) Dial(network, addr string, dialFn base.DialFunc, a
 	if err != nil {
 		return nil, err
 	}
+	dialConn := conn
 	if conn, err = newObfs2ClientConn(conn); err != nil {
-		conn.Close()
+		dialConn.Close()
 		return nil, err
 	}
 	return conn, nil

transports/obfs3/obfs3.go

@@ -97,8 +97,9 @@ func (cf *obfs3ClientFactory) Dial(network, addr string, dialFn base.DialFunc, a
 	if err != nil {
 		return nil, err
 	}
+	dialConn := conn
 	if conn, err = newObfs3ClientConn(conn); err != nil {
-		conn.Close()
+		dialConn.Close()
 		return nil, err
 	}
 	return conn, nil

transports/scramblesuit/base.go

@@ -87,8 +87,9 @@ func (cf *ssClientFactory) Dial(network, addr string, dialFn base.DialFunc, args
 	if err != nil {
 		return nil, err
 	}
+	dialConn := conn
 	if conn, err = newScrambleSuitClientConn(conn, cf.ticketStore, ca); err != nil {
-		conn.Close()
+		dialConn.Close()
 		return nil, err
 	}
 	return conn, nil