driver model: anti-oopsing medicine

Anti-oops medicine for the class iterators ... the oops was
observed when a class was implicitly referenced before it
was initialized.

[Modified by Greg to spit a warning back so someone knows to fix their code]
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/class.c

@@ -295,6 +295,12 @@ int class_for_each_device(struct class *class, struct device *start,
 
 	if (!class)
 		return -EINVAL;
+	if (!class->p) {
+		WARN(1, "%s called for class '%s' before it was initialized",
+		     __func__, class->name);
+		return -EINVAL;
+	}
+
 	mutex_lock(&class->p->class_mutex);
 	list_for_each_entry(dev, &class->p->class_devices, node) {
 		if (start) {
@@ -344,6 +350,11 @@ struct device *class_find_device(struct class *class, struct device *start,
 
 	if (!class)
 		return NULL;
+	if (!class->p) {
+		WARN(1, "%s called for class '%s' before it was initialized",
+		     __func__, class->name);
+		return NULL;
+	}
 
 	mutex_lock(&class->p->class_mutex);
 	list_for_each_entry(dev, &class->p->class_devices, node) {