batman-adv: Only increase refcounter once for alternate router

The test whether we can use a router for alternating bonding should only be
done once because it is already known that it is still usable and will not be
deleted from the list soon.

This patch addresses Coverity #712285: Unchecked return value

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>