Fixes panic: runtime error: index out of range
goroutine 10 [running]:
testing.func·006()
/usr/local/go/src/testing/testing.go:441 +0x181
github.com/shirou/gopsutil/docker.CgroupMem(0x586b30, 0x6, 0x5a87d0, 0x1c, 0x0, 0x0, 0x0)
/home/jwilder/go/src/github.com/shirou/gopsutil/docker/docker_linux.go:119 +0xf48
github.com/shirou/gopsutil/docker.CgroupMemDocker(0x586b30, 0x6, 0x0, 0x0, 0x0)
/home/jwilder/go/src/github.com/shirou/gopsutil/docker/docker_linux.go:184 +0x57
If the ID passed to the CGroupMemDocker does not exist, you can get
a panic at runtime. This can happen when a container exits before
calling the func.