net: Fix NULL pointer dereference when parsing ICMP6_ROUTER_ADVERTISE messages

net: Fix NULL pointer dereference when parsing ICMP6_ROUTER_ADVERTISE messages

During UEFI PXE boot in IPv6 network, if the DHCP server adopts stateful
automatic configuration, then the client receives a ICMP6_ROUTER_ADVERTISE
multicast message from the server. This may be received without the interface
having a configured network address, so orig_inf will be NULL, which can lead
to a NULL dereference when creating the default route. Actually, in this case,
the client obtains the default route through DHCPv6 instead of RA messages.
So if orig_inf == NULL and route_inf == NULL, we should not set the
default route.

Fixes: https://savannah.gnu.org/bugs/?62072
Signed-off-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>