Get or Getw on a Null Node causes lock

[Aaron Raddon]

issuing a Get() or Getw() on a valid zookeeper path which conatains a Null nodes locks up.

This node from zkcli locks it up

[zk: localhost:2181(CONNECTED) 2] get /brokers/topics/test
null
cZxid = 0xa
ctime = Thu Feb 23 00:40:17 UTC 2012
mZxid = 0xa
mtime = Thu Feb 23 00:40:17 UTC 2012

I fixed it in my branch, was going to submit a test but couldn't get the test suite running.