netfilter: nft_set_pipapo: don't return bogus extension pointer

Dan Carpenter says:
Commit 17a20e09 ("netfilter: nft_set: remove one argument from
lookup and update functions") [..] leads to the following Smatch
static checker warning:

 net/netfilter/nft_set_pipapo_avx2.c:1269 nft_pipapo_avx2_lookup()
 error: uninitialized symbol 'ext'.

Fix this by initing ext to NULL and set it only once we've found
a match.

Fixes: 17a20e09 ("netfilter: nft_set: remove one argument from lookup and update functions")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/netfilter-devel/aJBzc3V5wk-yPOnH@stanley.mountain/


Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>