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>