Fix bug in verifying directory signatures with short digests

If we got a signed digest that was shorter than the required digest
length, but longer than 20 bytes, we would accept it as long
enough.... and then immediately fail when we want to check it.

Fixes bug 2409; bug in 0.2.2.20-alpha; found by piebeer.

src/or/routerparse.c

@@ -1088,7 +1088,7 @@ check_signature_token(const char *digest,
   signed_digest = tor_malloc(keysize);
   if (crypto_pk_public_checksig(pkey, signed_digest, keysize,
                                 tok->object_body, tok->object_size)
-      < DIGEST_LEN) {
+      < digest_len) {
     log_warn(LD_DIR, "Error reading %s: invalid signature.", doctype);
     tor_free(signed_digest);
     return -1;