sctp: remove unreachable code from sctp_sf_violation_chunk()

sctp_sf_violation_chunk() is not called with asoc argument equal to NULL,
but if that happens it would lead to NULL pointer dereference
in sctp_vtag_verify().

The patch removes code that handles NULL asoc in sctp_sf_violation_chunk().

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Proposed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>