NFT_JUMP/GOTO erronously sets length to sizeof(void *).

We then allocate insufficient memory when such element is added to a vmap.
Suggested-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>