Check sockaddr_len before dereferencing sp->sa_protocol, to ensure that
it actually points to valid data.
Fixes: fd558d186d ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Reported-by: [email protected]
Signed-off-by: Guillaume Nault <[email protected]>
Signed-off-by: David S. Miller <[email protected]>