library: guard against 'inf/nan' for 'cpu utilization'

library: guard against 'inf/nan' for 'cpu utilization'

When preparing the ps program for a %CUU field, it was
revealed the PIDS_UTILIZATION item may sometimes yield
a result of 'nan' or 'inf' for that ps program itself.

Apparently, it was caused by the short lived nature of
such a one-shot program. And, while this anomaly could
be handled in ps, the solution belongs in the library.

So, I reworked an item's algorithm to avoid this oops.

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