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

internal/ctlsock/ctlsock_serve.go

@@ -96,9 +96,9 @@ func (ch *ctlSockHandler) handleConnection(conn *net.UnixConn) {
 			conn.Close()
 			return
 		}
-		buf = buf[:n]
+		data := buf[:n]
 		var in RequestStruct
-		err = json.Unmarshal(buf, &in)
+		err = json.Unmarshal(data, &in)
 		if err != nil {
 			tlog.Warn.Printf("ctlsock: JSON Unmarshal error: %#v", err)
 			err = errors.New("JSON Unmarshal error: " + err.Error())
@@ -106,8 +106,6 @@ func (ch *ctlSockHandler) handleConnection(conn *net.UnixConn) {
 			continue
 		}
 		ch.handleRequest(&in, conn)
-		// Restore original size.
-		buf = buf[:cap(buf)]
 	}
 }