System Rules of Cpu Usage has a calculating bug,May occured Double NaN or Infinity returned

## Issue Description

Type: *bug report*

### Describe what you expected to happen
sentinel-core.jar com.alibaba.csp.sentinel.slots.system.SystemStatusListener
Sometimes get wrong cpu usage,then block the apps. When System Rule has been set.

### How to reproduce it (as minimally and precisely as possible)

1. sentinel version: 1.8.0
2. add a log of current cpu usage in com.alibaba.csp.sentinel.slots.system.SystemStatusListener
3. watch it soon you will see NaN of Infinity printed

### Tell us your environment
sentinel version: 1.8.0
os linux 64

Start with com.alibaba.csp.sentinel.slots.system.SystemStatusListener in sentinel-core.jar and reproduce the report on Linux 64 with Sentinel 1.8.0 while logging CPU usage. Done means CPU usage no longer produces NaN or Infinity and a configured system rule does not incorrectly block the application.

Written by the indexing model from the issue text.