Don't ignore err when getting CgroupMemDocker stats

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.

docker/docker_linux.go

@@ -109,7 +109,10 @@ func CgroupMem(containerid string, base string) (*CgroupMemStat, error) {
 	if len(containerid) == 0 {
 		containerid = "all"
 	}
-	lines, _ := common.ReadLines(path)
+	lines, err := common.ReadLines(path)
+	if err != nil {
+		return nil, err
+	}
 	ret := &CgroupMemStat{ContainerID: containerid}
 	for _, line := range lines {
 		fields := strings.Split(line, " ")

docker/docker_linux_test.go

@@ -44,3 +44,10 @@ func TestCgroupMem(t *testing.T) {
 		}
 	}
 }
+
+func TestCgroupMemInvalidId(t *testing.T) {
+	_, err := CgroupMemDocker("bad id")
+	if err == nil {
+		t.Error("Expected path does not exist error")
+	}
+}