xfrm: fix null pointer dereference on state and tmpl sort

Creating sub policy that matches the same outer flow as main policy does
leads to a null pointer dereference if the outer mode's family is ipv4.
For userspace compatibility, this patch just eliminates the crash i.e.,
does not introduce any new sorting rule, which would fruitlessly affect
all but the aforementioned case.

Signed-off-by: Koichiro Den <den@klaipeden.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>