Use unwatch between multi and exec, and throw NullPointException when executing Exec

[dismasson]

Expected behavior

Unwatch can be exec before execution

Actual behavior

Use unwatch between multi and exec, and throw NullPointException when executing Exec

Steps to reproduce:

    jedis.watch("key");
    Transaction transaction = jedis.multi();
    jedis.unwatch();
    transaction.set("key","value");
    transaction.exec();

Jedis version:

2.9.0

Java vcersion:

1.7

[sazzad16]

Resolved by #2031 and #2032