ovs: fail port enslavement when the bridge device is not found

Fail the enslavement of the ovs port if the bridge device is not
found, instead of generating assertions and potentially crash later.

https://bugzilla.redhat.com/show_bug.cgi?id=1797696

Fixes: 101e65d2bb ('ovs: allow changing mac address of bridges and interfaces')
(cherry picked from commit c5c49995b1)

src/devices/ovs/nm-device-ovs-port.c

@@ -103,6 +103,11 @@ enslave_slave (NMDevice *device, NMDevice *slave, NMConnection *connection, gboo
 	}
 
 	bridge_device = nm_active_connection_get_device (ac_bridge);
+	if (!bridge_device) {
+		_LOGW (LOGD_DEVICE, "can't enslave %s: bridge device not found",
+		       nm_device_get_iface (slave));
+		return FALSE;
+	}
 
 	nm_ovsdb_add_interface (nm_ovsdb_get (),
 	                        nm_active_connection_get_applied_connection (ac_bridge),