Make extend_info_from_node() more picky about node contents

This update is needed to make it consistent with the behavior of
node_awaiting_ipv6(), which doesn't believe in the addresses from
routerinfos unless it actually plans to use those routerinfos.

Fixes bug 25213; bugfix on b66b62fb75 in 0.3.3.1-alpha,
which tightened up the definition of node_awaiting_ipv6().

changes/bug25213

@@ -0,0 +1,5 @@
+  o Minor bugfixes (warnings, ipv6):
+    - Avoid a bug warning that could occur when trying to connect to
+      a relay over IPv6 on a Tor instance that downloads router descriptors,
+      but prefers to use microdescriptors. Fixes bug 25213; bugfix on
+      0.3.3.1-alpha.

src/or/circuitbuild.c

@@ -2857,8 +2857,18 @@ extend_info_from_node(const node_t *node, int for_direct_connect)
   tor_addr_port_t ap;
   int valid_addr = 0;
 
-  if (node->ri == NULL && (node->rs == NULL || node->md == NULL))
-    return NULL;
+  const int is_bridge = node_is_a_configured_bridge(node);
+  const int we_use_mds = we_use_microdescriptors_for_circuits(get_options());
+
+  if (is_bridge || !we_use_mds) {
+    /* We need an ri in this case. */
+    if (!node->ri)
+      return NULL;
+  } else {
+    /* Otherwise we need an md. */
+    if (node->rs == NULL || node->md == NULL)
+      return NULL;
+  }
 
   /* Choose a preferred address first, but fall back to an allowed address.
    * choose_address returns 1 on success, but get_prim_orport returns 0. */