{s,u}time values of a process wrongly decrease in /proc/<PID>/stat when a thread exits.

Issue

• top command displayed a 4500% CPU utilization on a 16-CPU system.
• pidstat shows bogus values
  When the system is running on high load, pidstat will report bogus values for process:

05:45:26 PM     29057    1.00    0.00    0.00    1.00     4  ssmagent.bin
05:45:27 PM     29057    0.00    1.00    0.00    1.00     4  ssmagent.bin
05:45:28 PM     29057    0.00 18446744073709551616.00    0.00 18446744073709551616.00     4  ssmagent.bin
05:45:29 PM     29057    0.00    1.00    0.00    1.00     4  ssmagent.bin
05:45:30 PM     29057    0.00    0.00    0.00    0.00     4  ssmagent.bin