fixed bug for Scrypt test vectors

Author: jprichardson

lib/pbkdf2.js

@@ -48,6 +48,7 @@ function pbkdf2(passwd, S, c, dkLen) {
         arraycopy(U, 0, T, 0, hLen);
 
         for (var j = 1; j < c; j++) {
+            sha256.init(passwd);
             sha256.update(U);
             U = sha256.finalize();