It's necessary to make this clamping there so that returned values are
always used consistently in taskmgr, including in places where the
values are directly fetched into other functions.
This is a free adaptation from
Wine commit
a9742b3210c4cec67aca3c0012f3b9504a4368cf
From: Gerald Pfeifer <gerald@pfeifer.com>
taskmgr: Move out-of-domain checking into
PerfDataGetProcessorUsage() and PerfDataGetProcessorSystemUsage().