batman-adv: Ensure that we really have route changes in update_route

The debug output of update_route has tests for "route deleted" and "route
added". All other situations are handled as "route changed". This is not
true because neigh_node and curr_router could be both NULL.

The function is not called in this situation, but the code might be
interpreted wrong when reading it without this test.

Signed-off-by: Sven Eckelmann <sven@narfation.org>