netfilter: nft_set_pipapo: do not rely on ZERO_SIZE_PTR

pipapo relies on kmalloc(0) returning ZERO_SIZE_PTR (i.e., not NULL
but pointer is invalid).

Rework this to not call slab allocator when we'd request a 0-byte
allocation.

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>