s390/unwind: fix get_stack_pointer(NULL, NULL)

unwind_for_each_frame(NULL, NULL, 0) does not return any valid frames.

The reason is that get_stack_pointer, unlike get_stack_info and
show_stack, does not handle NULL argument.

Fix by making get_stack_pointer treat NULL as current, like
get_stack_info and show_stack do.
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/stacktrace.h

@@ -38,7 +38,7 @@ static inline unsigned long get_stack_pointer(struct task_struct *task,
 {
 	if (regs)
 		return (unsigned long) kernel_stack_pointer(regs);
-	if (task == current)
+	if (!task || task == current)
 		return current_stack_pointer();
 	return (unsigned long) task->thread.ksp;
 }