rs485echo: fix segmentation fault in echo mode

Currently, rs485echo does not start in echo mode:

root@GFMM03731060:~# /opt/ltp/testcases/bin/rs485echo -e
Segmentation fault
root@GFMM03731060:~# 

This MR fixes that.

Note: as this is a fix for an application, we did not test pipeline images and did not collect logs/