check if seed is greater than 512 bits

lib/HierarchicalKey.js

@@ -63,6 +63,8 @@ HierarchicalKey.seed = function(bytes, network) {
     bytes = new Buffer(bytes, 'hex'); //if not buffer, assume hex
   if (bytes.length < 128/8)
     return false; //need more entropy
+  if (bytes.length > 512/8)
+    return false;
   var hash = coinUtil.sha512hmac(bytes, new Buffer('Bitcoin seed'));
 
   var hkey = new HierarchicalKey(null);