Avoid dereferencing NULL if a bridge fails to build an ei descriptor.

Reported by an anonymous commenter on Trac.

changes/fix2195-fix

@@ -0,0 +1,6 @@
+  o Minor bugfixes:
+    - Do not dereference NULL if a bridge fails to build its
+      extra-info descriptor. Previously, we would have dereferenced
+      NULL. Found by an anonymous commenter on Trac. Bugfix on
+      0.2.2.19-alpha.
+

src/or/router.c

@@ -1523,7 +1523,7 @@ router_rebuild_descriptor(int force)
   /* Let bridges serve their own descriptors unencrypted, so they can
    * pass reachability testing. (If they want to be harder to notice,
    * they can always leave the DirPort off). */
-  if (!options->BridgeRelay)
+  if (ei && !options->BridgeRelay)
     ei->cache_info.send_unencrypted = 1;
 
   router_get_router_hash(ri->cache_info.signed_descriptor_body,