0059-ps/output.c: Enforce a safe range for max_rightward.

0059-ps/output.c: Enforce a safe range for max_rightward.

Enforce a maximum max_rightward of OUTBUF_SIZE-1, because it is used in
constructs such as "snprintf(outbuf, max_rightward+1," (we could remove
the extra check at the beginning of forest_helper() now, but we decided
to leave it, as a precaution and reminder).

The minimum max_rightward check is not strictly needed, because it is
unsigned. However, we decided to add it anyway:

- most of the other variables are signed;

- make it visually clear that this case is properly handled;

- ideally, the minimum max_rightward should be 1, not 0 (to prevent
  integer overflows such as "max_rightward-1"), but this might change
  the behavior/output of ps, so we decided against it, for now.

Instead, we fixed the only function that overflows if max_rightward is
0. Also, enforce the same safe range for max_leftward, although it is
never used throughout the code-base.

---------------------------- adapted for newlib branch
. adapted via 'patch' without rejections

Signed-off-by: Jim Warner <james.warner@comcast.net>