uac: reset the ongoing registration flags in case of error in tm callback

- resulted in being stuck in a state that didn't retry registration
- reported by AndyJRobinson, GH#124

modules/uac/uac_reg.c

@@ -898,6 +898,7 @@ void uac_reg_tm_callback( struct cell *t, int type, struct tmcb_params *ps)
 	}
 
 error:
+	ri->flags &= ~(UAC_REG_ONGOING|UAC_REG_AUTHSENT);
 	if(reg_retry_interval) {
 		ri->timer_expires = time(NULL) + reg_retry_interval;
 	} else {