Fixes a Denial-of-Service vulnerability where a DPD R_U_THERE or R_U_THERE_ACK NOTIFY
message (RFC 3706, Dead Peer Detection) received on UDP port 500 or 4500 and not
related to an existing ISAKMP Security Association causes an immediate crash of the
IKEv1 pluto daemon while dereferencing a NULL state pointer.

Apply to strongSwan releases 4.0.0 to 4.2.13