Fix resource leak in parse_consensus_request()

We were allocating diff_hash_in_url on some URLs, but not freeing it.

Fixes CID 1409669.  Bug not in any released Tor.

src/or/consdiffmgr.c

@@ -566,6 +566,7 @@ consdiffmgr_find_consensus(struct consensus_cache_entry_t **entry_out,
                            consensus_flavor_t flavor,
                            compress_method_t method)
 {
+  tor_assert(entry_out);
   tor_assert((int)flavor < N_CONSENSUS_FLAVORS);
 
   int pos = consensus_compression_method_pos(method);