dccp: check sk for closed state in dccp_sendmsg()

dccp: check sk for closed state in dccp_sendmsg()

dccp_disconnect() sets 'dp->dccps_hc_tx_ccid' tx handler to NULL,
therefore if DCCP socket is disconnected and dccp_sendmsg() is
called after it, it will cause a NULL pointer dereference in
dccp_write_xmit().

This crash and the reproducer was reported by syzbot. Looks like
it is reproduced if commit 69c64866ce07 ("dccp: CVE-2017-8824:
use-after-free in DCCP code") is applied.

Reported-by: [email protected]
Signed-off-by: Alexey Kodanev <[email protected]>
Signed-off-by: David S. Miller <[email protected]>