dccp: check ccid before dereferencing

dccp: check ccid before dereferencing

ccid_hc_rx_getsockopt() and ccid_hc_tx_getsockopt() might be called with
a NULL ccid pointer leading to a NULL pointer dereference. This could
lead to a privilege escalation if the attacker is able to map page 0 and
prepare it with a fake ccid_ops pointer.

Signed-off-by: Mathias Krause <[email protected]>
Cc: Gerrit Renker <[email protected]>
Cc: [email protected]
Signed-off-by: David S. Miller <[email protected]>