Bluetooth: Mesh: Model extensions walk stops before last model

When reaching the last model in the circular extension linked list, the
walker would abandon the walk before checking the last model. This makes
us skip models when checking the subscription list, potentially causing
incoming messages to be wrongfully ignored.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>