pidstat: Now handle processes with spaces in their name properly

pidstat had a bug which made it unable to handle processes with spaces
in their name, eg:

$ cat /proc/5768/stat
5768 (Plex New Transc) S 1264 1252 1252 0 -1 1077960704 17317 0 18 0
9260 137 0 0 15 -5 8 0 1430749 186589184 16704 18446744073709551615 1 1
0 0 0 0 0 0 8404998 18446744073709551615 0 0 17 1 0 0 22 0 0 0 0 0 0 0 0
0 0

This patch fixes the problem.

Reported by Chris Grindstaff (@cgrinds).

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>