ctlsock: fix buffer truncation of JSON unmarshal error
ctlsock: fix buffer truncation of JSON unmarshal error

In the error case, buf was not restored to the original
capacity. Instead of truncating "buf" and restoring (or forgetting to restore)
later, introduce the "data" slice.

Fixes https://github.com/rfjakob/gocryptfs/issues/356

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs