Check answer_len in the remap_addr case of process_relay_cell_not_open.

Fix an edge case where a malicious exit relay could convince a
controller that the client's DNS question resolves to an internal IP
address. Bug found and fixed by "optimist"; bugfix on 0.1.2.8-beta.

ChangeLog

@@ -1,4 +1,9 @@
 Changes in version 0.2.2.1-alpha - 2009-??-??
+  o Security fixes:
+    - Fix an edge case where a malicious exit relay could convince a
+      controller that the client's DNS question resolves to an internal IP
+      address. Bug found and fixed by "optimist"; bugfix on 0.1.2.8-beta.
+
   o Major features:
     - Add support for dynamic OpenSSL hardware crypto acceleration engines
       via new AccelName and AccelDir options.

src/or/relay.c

@@ -947,7 +947,7 @@ connection_edge_process_relay_cell_not_open(
                    cell->payload+RELAY_HEADER_SIZE+2, /*answer*/
                    ttl,
                    -1);
-    if (answer_type == RESOLVED_TYPE_IPV4) {
+    if (answer_type == RESOLVED_TYPE_IPV4 && answer_len >= 4) {
       uint32_t addr = ntohl(get_uint32(cell->payload+RELAY_HEADER_SIZE+2));
       remap_event_helper(conn, addr);
     }