Fix possible buffer overflow

From dcb314:

[rd_stats.c:835]: (warning) scanf without field width limits can crash
with huge input data.

So changed "n = fscanf(fp, "%s", duplex);" with
"n = fscanf(fp, "%31s", duplex);".

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>