ps: Stop crash if using test fields

ps: Stop crash if using test fields

I'm not sure if anyone actually uses these things, but if you
selected test fields on the command line ps would crash.

$ ps/pscommand -o _left
Signal 11 (SEGV) caught by pscommand (3.3.11.877-0488).
/home/csmall/Projects/procps/procps/ps/.libs/pscommand:ps/display.c:66: please report this bug
Segmentation fault

Anyway, it doesn't now:
$ ps/pscommand -o pid,_left,_left2,_right,_unlimited 1
    PID LLLLLLLL L2L2L2L2 RRRRRRRRRRR U
      1 tty7     3270/tty4      59:59 [123456789-12345] <defunct>