fix the capacity of make disk on FreeBSD.

disk_freebsd.go

@@ -17,7 +17,7 @@ func DiskPartitions(all bool) ([]DiskPartitionStat, error) {
 		return ret, err
 	}
 
-	fs := make([]Statfs, 0, count)
+	fs := make([]Statfs, count)
 	_, err = Getfsstat(fs, MNT_WAIT)
 
 	for _, stat := range fs {

disk_test.go

@@ -22,7 +22,7 @@ func TestDisk_usage(t *testing.T) {
 
 func TestDisk_partitions(t *testing.T) {
 	ret, err := DiskPartitions(false)
-	if err != nil {
+	if err != nil || len(ret) == 0 {
 		t.Errorf("error %v", err)
 	}
 	empty := DiskPartitionStat{}
@@ -35,7 +35,7 @@ func TestDisk_partitions(t *testing.T) {
 
 func TestDisk_io_counters(t *testing.T) {
 	ret, err := DiskIOCounters()
-	if err != nil {
+	if err != nil || len(ret) == 0 {
 		t.Errorf("error %v", err)
 	}
 	empty := DiskIOCountersStat{}