payload: return early if dependency is not a payload expression

payload: return early if dependency is not a payload expression

 if (dep->left->payload.base != PROTO_BASE_TRANSPORT_HDR)

is legal only after checking that ->left points to an
EXPR_PAYLOAD expression. The dependency store can also contain
EXPR_META, in this case we access a bogus part of the union.

The payload_may_dependency_kill_icmp helper can't handle a META
dep either, so return early.

Fixes: 533565244d88 ("payload: check icmp dependency before removing previous icmp expression")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>