Fix logic errors in the zero-inode path. (#352)

Fix logic errors in the zero-inode path. (#352)

I've now tested the zero-inode path on a Wasm engine specially-modified
to have `fd_readdir` set inode numbers to zero. Fix two bugs this turned up:
 - Increment `buffer_processed`, as noticed by @yamt
 - Don't do an `fstatat` on "..", because that references a path outside
   of the directory, which gets a permission-denied error.