RTOS: Unify wipe-out of thread list

Each RTOS implementation uses it's own (similar) code to free
the thread list. There are some additional issues:

<--->
if (pointer != NULL)
	free(pointer);
<--->
This is not necessary, free(NULL) is perfectly ok.

<--->
free(rtos->thread_details);
rtos->thread_details = NULL;
rtos->thread_count = 0;
<--->
The 3rd line has been missing for all RTOS but ChibiOs. There are paths
in the code where rtos->thread_count is never set to NULL, which can
lead to null pointer dereference of rtos->thread_details.

Change-Id: I6f7045c3d4518b925cb80dd5c907a566536b34ad
Signed-off-by: Christian Eggers <ceggers@gmx.de>
---
Changelog:
v7:
- rtos_wipe_threadlist() --> rtos_free_threadlist()
- removed non related changes in gdb_server.c from this patch
v3:
- Removed world "topic" from first line of commit message
v2:
- typo: "whipe" --> "wipe"
Reviewed-on: http://openocd.zylin.com/1916


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>