rs485echo: fix segmentation fault in echo mode

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>

testcases/rs485echo/rs485echo.c

@@ -501,7 +501,7 @@ main(int   argc,    /* number of command line parameters                      */
 	// Perform global test setup
 	setup();
 
-	if ( fake_data_len > message->length)
+	if (!echo_mode && fake_data_len > message->length)
 		fake_data_len = message->length;
 
 	tst_resm(TINFO, "Starting RS485 echo test%s", echo_mode?" in echo mode.":".");