netfilter: xt_HMARK: potential NULL dereference in get_inner_hdr()

There is a typo in the error checking and "&&" was used instead of "||".
If skb_header_pointer() returns NULL then it leads to a NULL
dereference.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Hans Schillstrom <hans.schillstrom@ericsson.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>