netfilter: nft_range: add the missing NULL pointer check

Otherwise, kernel panic will happen if the user does not specify
the related attributes.

Fixes: 0f3cd9b3

 ("netfilter: nf_tables: add range expression")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>