Fix overflow of "running" value in getstat()

Fix overflow of "running" value in  getstat()

It seems in some cases procs_running field of /proc/stat can contain 0 even if vmstat itself is running. At least this can be reproduced on Linux 3.9.3 compiled with BFS scheduler.

Since getstat() decrements value of procs_running by 1, we get overflow:

$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0 667732 918996  57376 911260   21   30    36    40   98   45 14 82  4  1
 4294967295  0 667728 916716  57376 911264    8    0     8     0 1958 3733 28  7 65  1
 0  0 667700 915996  57376 911416   24    0   152     0 1735 3600 23  5 71  1
 4294967295  0 667700 915872  57376 911392    0    0     0     0 1528 3165 21  4 76  0