netfilter: nft_inner: fix IS_ERR() vs NULL check

The __nft_expr_type_get() function returns NULL on error.  It never
returns error pointers.

Fixes: 3a07327d

 ("netfilter: nft_inner: support for inner tunnel header matching")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>