bgpd: Show that prefix is malformed if aggregated by 0

Show if this malformed under `show [ip] bgp <prefix>`:
 ```
eva# sh ip bgp 103.79.124.0/22
BGP routing table entry for 103.79.124.0/22
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  192.168.201.136
  64539 15096 6939 7545 7545 136001, (aggregated by 0(malformed) 0.0.0.0)
    192.168.201.136 from 192.168.201.136 (192.168.201.136)
      Origin IGP, valid, external, best (First path received)
      Last update: Thu Mar 26 10:02:07 2020
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

bgpd/bgp_route.c

@@ -8713,10 +8713,22 @@ void route_vty_out_detail(struct vty *vty, struct bgp *bgp,
 			json_object_string_add(
 				json_path, "aggregatorId",
 				inet_ntoa(attr->aggregator_addr));
+			if (attr->aggregator_as == BGP_AS_ZERO)
+				json_object_boolean_true_add(
+					json_path, "aggregatorAsMalformed");
+			else
+				json_object_boolean_false_add(
+					json_path, "aggregatorAsMalformed");
 		} else {
-			vty_out(vty, ", (aggregated by %u %s)",
-				attr->aggregator_as,
-				inet_ntoa(attr->aggregator_addr));
+			if (attr->aggregator_as == BGP_AS_ZERO)
+				vty_out(vty,
+					", (aggregated by %u(malformed) %s)",
+					attr->aggregator_as,
+					inet_ntoa(attr->aggregator_addr));
+			else
+				vty_out(vty, ", (aggregated by %u %s)",
+					attr->aggregator_as,
+					inet_ntoa(attr->aggregator_addr));
 		}
 	}