avoid suicide

container.Kill() might read a pid of 0 from
container.State.Pid due to losing a race with
container.monitor() calling
container.State.SetStopped(). Sending a SIGKILL to
pid 0 is undesirable as "If pid equals 0, then sig
is sent to every process in the process group of
the calling process."

Docker-DCO-1.1-Signed-off-by: Daniel Norberg <daniel.norberg@gmail.com> (github: danielnorberg)
Merge pull request #5342 from danielnorberg/avoid-suicide

avoid suicide