netfilter: nf_conntrack_sip: validate skb_dst() before accessing it

In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_conntrack_sip: validate skb_dst() before accessing it tc ingress and openvswitch do not guarantee routing information to be available. These subsystems use the conntrack helper infrastructure, and the SIP helper relies on the skb_dst() to be present if sip_external_media is set to 1 (which is disabled by default as a module parameter). This effectively disables the sip_external_media toggle for these subsystems without resulting in a crash.