task: Destroy zero mars context ptr

This patch zero's the task id MARS context pointer to invalidate the task id
when before a task is created and after it is destroyed to catch some cases
where the user tries to use an invalid task id.

Signed-off-by: Yuji Mano <yuji.mano@am.sony.com>
Acked-by: Kazunori Asayama <asayama@sm.sony.co.jp>