Make AES reset a no-op after first call, since data doesn't change.

The AES _doReset function sets up a bunch of structures that aren't
actually modified when the cipher runs.  We can save a bunch of time by
re-using previously set up data on subsequent calls to _doReset.

src/aes.js

@@ -76,6 +76,8 @@
      */
     var AES = C_algo.AES = BlockCipher.extend({
         _doReset: function () {
+            if (this._nRounds) return;
+
             // Shortcuts
             var key = this._key;
             var keyWords = key.words;