gozk crashes when trying to get null data

[Alexey Borzenkov]

When node has NULL data gozk crashes in Conn.Get(). The reason is that in such cases zoo_wget returns -1 in buffer_len, which GoStringN interprets as a very huge string size, and which ultimately leads to a crash. Since by default ZooKeeper has at least one node that has NULL data it's very easy to get a crash by enumerating all nodes and calling Conn.Get for all of them. The patch I attached should fix the problem.