rtos: add sanity checking for FreeRTOS's quantity of priorities

rtos: add sanity checking for FreeRTOS's quantity of priorities

On operating systems with opportunistic malloc() (e.g. default setting in
GNU/Linux) malloc can sometimes allocate a huge memory region but later the
process will get killed on the first attempt to use this memory, so
checking for malloc's return value is not enough to prevent a crash.

This patch is compile-tested only.

Change-Id: I5e21663115c8e9a0ca9f3d71f7ba4bd09e5c3bb1
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/521
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>