[DECNET]: net/decnet/dn_route.c: fix inconsequent NULL checking

The Coverity checker noted this inconsequent NULL checking in
dnrt_drop().

Since all callers ensure that NULL isn't passed, we can simply remove
the check.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David S. Miller <davem@davemloft.net>