l3cfg: fix assertion failure
Fix the following assertion failure:
src/core/nm-l3cfg.c:2636:_l3_acd_data_state_change: assertion failed: (!acd_data->nacd_probe)
When AcdData enters state NM_L3_ACD_ADDR_STATE_READY, the duplicate address detection procedure completed successfully but the address is not configured yet on the interface. In the READY state we don't clear the probe because the same probe can be reused also for defending the address. Change the assertion.