Fix a check for when to rebuild the microdesc cache. (Backport from 0.2.3.

changes/bug2230_part2

+  o Minor bugfixes
+    - The microdesc journal is supposed to get rebuilt only if it is
+      at least _half_ the length of the store, not _twice_ the length
+      of the store.   Bugfix on 0.2.2.6-alpha; fixes part of bug 2230.
+

src/or/microdesc.c

@@ -208,7 +208,7 @@ microdescs_add_list_to_cache(microdesc_cache_t *cache,
     size_t old_content_len =
       cache->cache_content ? cache->cache_content->size : 0;
     if (cache->journal_len > 16384 + old_content_len &&
-        cache->journal_len > old_content_len * 2) {
+        cache->journal_len > old_content_len / 2) {
       microdesc_cache_rebuild(cache);
     }
   }