library: beef up 'enum pids_item' parameter validation

library: beef up 'enum pids_item' parameter validation

I was surprised to find that ol' gcc silently converts
a single (different) enum into an address where one or
more enums were expected to be dereferenced. Of course
this was just yet another way to generate an old SEGV.

So this commit will strengthen those parameter checks.

[ we will *not* blame Craig for a failure to consult ]
[ the documentation, since it doesn't even exist yet ]

Reference(s):
http://www.freelists.org/post/procps/newlib-ps-fix,8

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