net: fix page fault in route_cache::lookup() when no route is found

Fixes #475.

When there is no route, in_rtalloc_ign() sets ro.ro_rt to NULL. This
caused the following memcpy() to page fault:

  memcpy(ret, ro.ro_rt, sizeof(*ret));

This change makes lookup() exit early on failure and propagate the
information back to the caller.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>