rtos: Destroy RTOS and fix memory leak

rtos: Destroy RTOS and fix memory leak

The memory leak can be reproduced by using an arbitrary RTOS
and valgrind:

 $ valgrind --leak-check=full --show-leak-kinds=all

[...]
==9656== 224 (80 direct, 144 indirect) bytes in 1 blocks are definitely lost in loss record 3 of 3
==9656==    at 0x483CD99: calloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==9656==    by 0x1C541A: os_alloc (rtos.c:79)
==9656==    by 0x1C569E: os_alloc_create (rtos.c:111)
==9656==    by 0x1C569E: rtos_create (rtos.c:153)
==9656==    by 0x1AE332: target_configure (target.c:4899)
==9656==    by 0x1AF228: jim_target_configure (target.c:4952)
==9656==    by 0x1C9EF9: command_unknown (command.c:1066)
==9656==    by 0x313284: JimInvokeCommand (jim.c:10364)
==9656==    by 0x313FB6: Jim_EvalObj (jim.c:10814)
==9656==    by 0x3154A3: Jim_EvalFile (jim.c:11207)
==9656==    by 0x316015: Jim_SourceCoreCommand (jim.c:15230)
==9656==    by 0x313284: JimInvokeCommand (jim.c:10364)
==9656==    by 0x313B8B: JimEvalObjList (jim.c:10605)
[...]

Change-Id: I2cd41a154fb8570842601ff4e3e76502f5908f49
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: http://openocd.zylin.com/5479
Tested-by: jenkins
Reviewed-by: Moritz Fischer <moritzf@google.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>