It's possible for data->sock to get torn down in tcp error handling.
If we unconditionally tear it down again we will end up doing writes
to an offset of the NULL pointer when we go to tear it down again.

Detect if it has been torn down and don't do it again.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>