fix: resolve index out of bounds in diskio stat (#260)

## What does this PR do?

it's possible on linux to get an empty slice if the file is empty

protect against that by checking the slice len.

It's impossible on darwin but let's not rely on implementation details
and also check there.

## Why is it important?

prevent index out of bounds error

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## Related issues

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Author: kruskall

metric/cpu/metrics_darwin.go

@@ -44,7 +44,11 @@ func Get(m *Monitor) (CPUMetrics, error) {
 	for _, cpu := range perCPU {
 		cpulist = append(cpulist, fillCPU(cpu))
 	}
-	return CPUMetrics{totals: fillCPU(sum[0]), list: cpulist}, nil
+	cpuTotal := CPU{}
+	if len(sum) > 0 {
+		cpuTotal = fillCPU(sum[0])
+	}
+	return CPUMetrics{totals: cpuTotal, list: cpulist}, nil
 }
 
 func fillCPU(raw cpu.TimesStat) CPU {

metric/system/diskio/diskstat_linux.go

@@ -58,7 +58,9 @@ func (stat *IOStat) OpenSampling() error {
 	if err != nil {
 		return err
 	}
-	stat.curCPU = times[0]
+	if len(times) > 0 {
+		stat.curCPU = times[0]
+	}
 	return nil
 }