wireguard: prevent segfault on non-adj packets

wireguard: prevent segfault on non-adj packets

An unexpected packet that shows up on a Wireguard interace
that happens not to have a forwarding peer will cause a
segfault trying to index the vector of peers by adjacency.
Rather than segfaulting, recognize a non-adjacent packet
and drop it instead.

This leaves open the question of what _should_ be
happening to, say, IPv6 multicast packets.

Signed-off-by: Jon Loeliger <jdl@netgate.com>
Type: fix
Fixes: edca1325cf296bd0f5ff422fc12de2ce7a7bad88
Change-Id: Ic0a29e6cf6fe812a4895ec11bedcca86c62e590b