rtos: fix segfault in FreeRTOS handling

rtos: fix segfault in FreeRTOS handling

When gdb loads an elf file of a newer or older version of the firmware
being debugged, or when the firmware is not running yet, there's a high
probability of FreeRTOS variables to be read incorrectly, thus leading to
an attempt to allocate an enourmous amount of memory. Without this check
OpenOCD simply crashes and that's mad confusing.

Change-Id: I404a072e886d2d47d9d942cfaea8417eb8bd4a5d
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/520
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>