selinux: check length properly in SCTP bind hook

selinux_sctp_bind_connect() must verify if the address buffer has
sufficient length before accessing the 'sa_family' field. See
__sctp_connect() for a similar check.

The length of the whole address ('len') is already checked in the
callees.

Reported-by: Qian Cai <cai@gmx.us>
Fixes: d452930f ("selinux: Add SCTP support")
Cc: <stable@vger.kernel.org> # 4.17+
Cc: Richard Haines <richard_c_haines@btinternet.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Tested-by: Qian Cai <cai@gmx.us>
Signed-off-by: Paul Moore <paul@paul-moore.com>