netfilter: nf_tables: check for NULL in nf_tables_newchain pcpu stats allocation

alloc_percpu returns NULL on failure, not a negative error code.

Fixes: ff3cd7b3

 ("netfilter: nf_tables: refactor chain statistic routines")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>