Initialize trie variable

util.c

@@ -1505,7 +1505,7 @@ static char *blank_merkel = "000000000000000000000000000000000000000000000000000
 static bool parse_notify(struct pool *pool, json_t *val)
 {
   char *job_id, *prev_hash, *coinbase1, *coinbase2, *bbversion, *nbit,
-       *ntime, *header, *trie;
+       *ntime, *header, *trie = NULL;
   size_t cb1_len, cb2_len, alloc_len, header_len;
   unsigned char *cb1, *cb2;
   bool clean, ret = false, has_trie = false;