drbd: use list_first_entry_or_null in first_peer_device/first_connection

If there are no peer_devices or connections, I'd rather have NULL
than some "arbitrary" address pretending to point to a struct.

Helps to avoid hard to debug symptoms, in case we ever try to use
and dereference a drbd_connection or drbd_peer_device
where we in fact don't have any connection at all.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/drbd/drbd_int.h

@@ -853,7 +853,7 @@ static inline struct drbd_device *minor_to_device(unsigned int minor)
 
 static inline struct drbd_peer_device *first_peer_device(struct drbd_device *device)
 {
-	return list_first_entry(&device->peer_devices, struct drbd_peer_device, peer_devices);
+	return list_first_entry_or_null(&device->peer_devices, struct drbd_peer_device, peer_devices);
 }
 
 #define for_each_resource(resource, _resources) \
@@ -2222,7 +2222,7 @@ static inline void drbd_md_flush(struct drbd_device *device)
 
 static inline struct drbd_connection *first_connection(struct drbd_resource *resource)
 {
-	return list_first_entry(&resource->connections,
+	return list_first_entry_or_null(&resource->connections,
 				struct drbd_connection, connections);
 }