processes plugin: Fix handling of Linux 2.4.

Hi Florian et al

Thanks for the reply, but we detect a minor bug in the previous patch
due to kernel 2.4

The correct patch is attached. The bug is related with kernels 2.4,
where task/ directory do not exists and ps_read_task return -1, which is
catched and raise an error (breaking the ps_read_process function), so a
NaN is dispatched istead of values (number of process:1, number of
threads :1).

Sorry for the inconvenience :(

Regards,
  Andres

Signed-off-by: Florian Forster <octo@leeloo.lan.home.verplant.org>