netfilter: nft_set_pipapo: fix null deref for empty set

Blamed commit broke the check for a null scratch map:
  -  if (unlikely(!m || !*raw_cpu_ptr(m->scratch)))
  +  if (unlikely(!raw_cpu_ptr(m->scratch)))

This should have been "if (!*raw_ ...)".
Use the pattern of the avx2 version which is more readable.

This can only be reproduced if avx2 support isn't available.

Fixes: d8d871a3 ("netfilter: nft_set_pipapo: merge pipapo_get/lookup")
Signed-off-by: Florian Westphal <fw@strlen.de>