Fix panic in CgroupCPU

If an invalid container ID is passed to CgroupCPU,
a panic can result.

docker/docker_linux.go

@@ -71,7 +71,10 @@ func CgroupCPU(containerid string, base string) (*cpu.CPUTimesStat, error) {
 	}
 	path := path.Join(base, containerid, "cpuacct.stat")
 
-	lines, _ := common.ReadLines(path)
+	lines, err := common.ReadLines(path)
+	if err != nil {
+		return nil, err
+	}
 	// empty containerid means all cgroup
 	if len(containerid) == 0 {
 		containerid = "all"

docker/docker_linux_test.go

@@ -31,6 +31,13 @@ func TestCgroupCPU(t *testing.T) {
 	}
 }
 
+func TestCgroupCPUInvalidId(t *testing.T) {
+	_, err := CgroupCPUDocker("bad id")
+	if err == nil {
+		t.Error("Expected path does not exist error")
+	}
+}
+
 func TestCgroupMem(t *testing.T) {
 	v, _ := GetDockerIDList()
 	for _, id := range v {