monitor: missing cache and set handle initialization

monitor: missing cache and set handle initialization

This leads to a crash when adding stateful expressions to sets:

netlink.c:928:38: runtime error: member access within null pointer of type 'struct nft_ctx'
AddressSanitizer:DEADLYSIGNAL
=================================================================
==13781==ERROR: AddressSanitizer: SEGV on unknown address 0x0000000000d0 (pc 0x7fc96fc2b6b2 bp 0x7ffc0e26b080 sp 0x7ffc0e26b020 T0)
==13781==The signal is caused by a READ memory access.
==13781==Hint: address points to the zero page.
    #0 0x7fc96fc2b6b2 in table_cache_find /home/pablo/devel/scm/git-netfilter/nftables/src/cache.c:456
    #1 0x7fc96fd244d4 in netlink_parse_set_expr /home/pablo/devel/scm/git-netfilter/nftables/src/netlink_delinearize.c:1857
    #2 0x7fc96fcf1b4d in netlink_delinearize_set /home/pablo/devel/scm/git-netfilter/nftables/src/netlink.c:928
    #3 0x7fc96fd41966 in netlink_events_cache_addset /home/pablo/devel/scm/git-netfilter/nftables/src/monitor.c:649

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>