datatype: initialize TYPE_CT_LABEL slot in datatype array

datatype: initialize TYPE_CT_LABEL slot in datatype array

Otherwise, ct label with concatenations such as:

 table ip x {
        chain y {
                ct label . ct mark  { 0x1 . 0x1 }
        }
 }

crashes:

../include/datatype.h:196:11: runtime error: member access within null pointer of type 'const struct datatype'
AddressSanitizer:DEADLYSIGNAL
=================================================================
==640948==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fc970d3199b bp 0x7fffd1f20560 sp 0x7fffd1f20540 T0)
==640948==The signal is caused by a READ memory access.
==640948==Hint: address points to the zero page.
sudo     #0 0x7fc970d3199b in datatype_equal ../include/datatype.h:196

Fixes: 2fcce8b0677b ("ct: connlabel matching support")
Reported-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>