sctp: check af before verify address in sctp_addr_id2transport

sctp: check af before verify address in sctp_addr_id2transport

BugLink: http://bugs.launchpad.net/bugs/1702863
[ Upstream commit 912964eacb111551db73429719eb5fadcab0ff8a ]

Commit 6f29a1306131 ("sctp: sctp_addr_id2transport should verify the
addr before looking up assoc") invoked sctp_verify_addr to verify the
addr.

But it didn't check af variable beforehand, once users pass an address
with family = 0 through sockopt, sctp_get_af_specific will return NULL
and NULL pointer dereference will be caused by af->sockaddr_len.

This patch is to fix it by returning NULL if af variable is NULL.

Fixes: 6f29a1306131 ("sctp: sctp_addr_id2transport should verify the addr before looking up assoc")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>