Never choose a bridge as an exit. Bug 5342.

changes/bug5342

@@ -0,0 +1,3 @@
+  o Security fixes:
+    - Never use a bridge as an exit, even if it claims to be one.  Found by
+      wanoskarnet. Fixes bug 5342. Bugfix on ????.

src/or/circuitbuild.c

@@ -2704,7 +2704,11 @@ choose_good_exit_server_general(routerlist_t *dir, int need_uptime,
       n_supported[i] = -1;
       continue; /* skip routers that are known to be down or bad exits */
     }
-
+    if (router->purpose != ROUTER_PURPOSE_GENERAL) {
+      /* never pick a non-general node as a random exit. */
+      n_supported[i] = -1;
+      continue;
+    }
     if (options->_ExcludeExitNodesUnion &&
         routerset_contains_router(options->_ExcludeExitNodesUnion, router)) {
       n_supported[i] = -1;