Improve how TimerDemo.c manages differences between the tick count and its own intern...

Improve how TimerDemo.c manages differences between the tick count and its own internal tick count, which can temporarily differ when the tick hook is called while the scheduler is suspended.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@2191 1d2547de-c912-0410-9cb9-b8ca96c0e9e2