driver core: prevent device_for_each_child from oopsing

David Vrabel noticed that the wireless usb stack likes to call
device_for_each_chile() with an empty bus.  This used to work fine, but
now oopses.  This patch fixes the oops and makes the code behave like it
used to.
Reported-by: David Vrabel <david.vrabel@csr.com>
Tested-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/core.c

@@ -1142,6 +1142,9 @@ int device_for_each_child(struct device *parent, void *data,
 	struct device *child;
 	int error = 0;
 
+	if (!parent->p)
+		return 0;
+
 	klist_iter_init(&parent->p->klist_children, &i);
 	while ((child = next_device(&i)) && !error)
 		error = fn(child, data);