sctp: sctp_sock_filter(): avoid list_entry() on possibly empty list

sctp: sctp_sock_filter(): avoid list_entry() on possibly empty list

Use list_is_first() to check whether tsp->asoc matches the first
element of ep->asocs, as the list is not guaranteed to have an entry.

Fixes: 8f840e47f190 ("sctp: add the sctp_diag.c file")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20230208-sctp-filter-v2-1-6e1f4017f326@diag.uniroma1.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>