netfilter: xt_CT: fix unset return value if conntrack zone are disabled

net/netfilter/xt_CT.c: In function ‘xt_ct_tg_check_v1’:
net/netfilter/xt_CT.c:250:6: warning: ‘ret’ may be used uninitialized in this function [-Wmaybe-uninitialized]
net/netfilter/xt_CT.c: In function ‘xt_ct_tg_check_v0’:
net/netfilter/xt_CT.c:112:6: warning: ‘ret’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Reported-by: Borislav Petkov <bp@alien8.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>