map-agent: return NULL in case bssid not found in fhlist.

src/core/agent_map.c

@@ -637,6 +637,8 @@ struct netif_fh *wifi_get_netif_by_bssid(struct agent *a, uint8_t *bssid)
 		if (hwaddr_equal(fh->bssid, bssid))
 			return fh;
 	}
+
+	return NULL;
 }
 
 int wifi_get_radio_index_by_mac(struct agent *a,