Don't crash when reading cached*consensus files on startup

Fixes bug 1352

changes/bug1532

@@ -0,0 +1,4 @@
+ o Major bugfixes
+   - Tor directory authorities no longer crash when started with a
+     cached-microdesc-consensus file in their data directory. Bugfix on
+     0.2.2.6-alpha, fixes bug 1532.

src/or/dirserv.c

@@ -1597,6 +1597,8 @@ dirserv_get_runningrouters(void)
 cached_dir_t *
 dirserv_get_consensus(const char *flavor_name)
 {
+  if (!cached_consensuses)
+    return NULL;
   return strmap_get(cached_consensuses, flavor_name);
 }