[nrf fromtree] bluetooth: mesh: zeroing not used part of auth array

Commit set zero to not used part of authentication array
to prevent garbage from the previous provisioning using.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>
(cherry picked from commit 760d190)
Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>

Author: alxelax

subsys/bluetooth/mesh/prov.c

@@ -293,6 +293,7 @@ int bt_mesh_input_string(const char *str)
 	}
 
 	memcpy(bt_mesh_prov_link.auth, str, strlen(str));
+	memset(bt_mesh_prov_link.auth + size, 0, sizeof(bt_mesh_prov_link.auth) - size);
 
 	bt_mesh_prov_link.role->input_complete();