ps: handle incorrect -D values

I just noticed a slight issue with the '-D' option. Basically,
I accidentally typed something along the lines of:

ps -Dboth-p303

I.E. missing out the "space". Instead of giving an error, it behaved as
if I'd just typed "ps".

Looking at bin/ps/ps.c, where the -D option is parsed, it doesn't
error-out if there is no valid match.

This commit fixes the bug.