Report deleted consensuses as NOT_FOUND rather than AVAILABLE

This bug happened because of a bogus pointer check in
consdiffmgr_find_consensus(), not in any released Tor.

Fixes CID 1409670.  Good catch, Coverity!

src/or/consdiffmgr.c

@@ -577,7 +577,7 @@ consdiffmgr_find_consensus(struct consensus_cache_entry_t **entry_out,
   if (!handle)
     return CONSDIFF_NOT_FOUND;
   *entry_out = consensus_cache_entry_handle_get(handle);
-  if (entry_out)
+  if (*entry_out)
     return CONSDIFF_AVAILABLE;
   else
     return CONSDIFF_NOT_FOUND;