decnet: fix using plain integer as NULL warning

Fixes the following sparse warning:
net/decnet/dn_route.c:407:30: warning: Using plain integer as NULL pointer
net/decnet/dn_route.c:1923:22: warning: Using plain integer as NULL pointer

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>