Comment 16 for bug 1905159

Replace the 3rd arg of strncpy by 'mx' - it compiles without warning,
maybe it becomes less optimal, as end of the source string is to be
detected twice (by strlen and by strncpy) instead once.

But my attempts to get the "output truncated" warning in a simple
program (to find what caused it in EPICS) remain unsuccesful.