Potential Segmentation Fault fixed

If I see it right there could be a segmentation fault if used in echo mode. When you choose echo mode the variable message wouldn't a value assigned and in the cleanup function the not allocated memory would be free'ed.

testcases/rs485echo/rs485echo.c

@@ -95,7 +95,7 @@ int  fd;                             /* Handle for open serial port           */
 char* device_name = "/dev/ttymxc2";  /* String containing the device name     */
 struct termios termios;              /* termios structure for serial port     */
 int bytes = 8;                       /* Number of bytes to be sent            */
-struct message *message;             /* Message to be sent                    */
+struct message *message = NULL;      /* Message to be sent                    */
 int size_of_message;                 /* Size of message                       */
 char* received_data = NULL;          /* Received string                       */
 int repetitions = 1;                 /* Number of repetitions                 */