osmo-mslookup-client: fix segfault for respond_error() caller

respond_error() passes r == 0. On error, consider it the last response for that
request.

Change-Id: I1acb0b8aa00c098d1f1f1cc17035daa38ce29fd3
diff --git a/src/mslookup/osmo-mslookup-client.c b/src/mslookup/osmo-mslookup-client.c
index 4a9e587..a200a7e 100644
--- a/src/mslookup/osmo-mslookup-client.c
+++ b/src/mslookup/osmo-mslookup-client.c
@@ -584,11 +584,11 @@
 	llist_for_each_entry_safe(c, n, &globals.socket_clients, entry) {
 		if (!strcmp(query_str, c->query_str)) {
 			socket_client_respond_result(c, g_buf);
-			if (r->last)
+			if (!r || r->last)
 				socket_client_close(c);
 		}
 	}
-	if (r->last)
+	if (!r || r->last)
 		globals.requests_handled++;
 }