netfilter: nf_tables: validate NFTA_SET_TABLE parameter

If the NFTA_SET_TABLE parameter is missing and the NLM_F_DUMP flag is
not set, then a NULL pointer dereference is triggered in
nf_tables_set_lookup because ctx.table is NULL.

Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>