In hostapd_ubus_add_bss(), ubus objects are not registered for mesh
interfaces. This provokes a segfault when accessing the ubus object in
mesh deinit.
This commit adds the same condition to hostapd_ubus_free_bss() for
discarding those mesh interfaces.
Signed-off-by: Jesus Fernandez Manzano <jesus.manzano@galgus.net>