set default return value for  `_deserialize_nonleaf_from_disk()`

hi Bohu, 

nessDB build failed on my OS X 10.9 with the following error message: https://gist.github.com/anonymous/8999425/raw, it shows that `r` in `_deserialize_nonleaf_from_disk()` wasn't initialised on some goto branches. this pr set up a default value of `NESS_ERR` for it. 

regards