sctp: Check address length before reading address family

KMSAN will complain if valid address length passed to connect() is shorter
than sizeof("struct sockaddr"->sa_family) bytes.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>