RTOS support: Correction of out of bound access of thread array

RTOS support: Correction of out of bound access of thread array

FreeRTOS use an array to store ready task. The array size is
configMAX_PRIORITIES. In the current implementation, the code try to access 1 more priority if the helper from freeRTOS contrib is used.
This has effect of detecting bad thread. This patch correct this and have been tested on
a code with more than 12 task.

Change-Id: Id229f0b2c4bf1aab87a2a69be174cc9b6dda00cb
Signed-off-by: Jonathan Dumaresq <jdumaresq@cimeq.qc.ca>
Reviewed-on: http://openocd.zylin.com/3400
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>