Be more defensive in native extension. (#10)
Be more defensive in native extension. (#10)

* Be more defensive in native extension.

- Explicitly reserve a byte for null terminator in strings
- Only close the connection being worked on with sasl_dispose rather than all of them

* Add some comments

* Review feedback

mongodb/mongo-ruby-kerberos,mongodb/mongo-ruby-kerberos,mongodb/mongo-ruby-kerberos