rtos: better sanity checking for uCOS-III

This patch improves the OSRunning check. If the rtos_running check
fails, update_threads will return an error rather than attempt to update
the thread list using bad values.

Change-Id: I8614c325504d3a9ab19aebb6862b1fe445a0c8e7
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/4166


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>