[freebsd] fix SwapMemory for hosts without swap

mem/mem_freebsd.go

@@ -91,7 +91,6 @@ func SwapMemory() (*SwapMemoryStat, error) {
 	if err != nil {
 		return nil, err
 	}
-	var ret *SwapMemoryStat
 	for _, line := range strings.Split(string(out), "\n") {
 		values := strings.Fields(line)
 		// skip title line
@@ -117,13 +116,13 @@ func SwapMemory() (*SwapMemoryStat, error) {
 			return nil, err
 		}
 
-		ret = &SwapMemoryStat{
+		return &SwapMemoryStat{
 			Total:       total_v,
 			Used:        used_v,
 			Free:        free_v,
 			UsedPercent: up_v,
-		}
+		}, nil
 	}
 
-	return ret, nil
+	return nil, errors.New("no swap devices found")
 }