batman-adv: don't pass a NULL hard_iface to batadv_hardif_put

In the case where hard_iface is NULL, the error path may pass a null
pointer to batadv_hardif_put causing a null pointer dereference error.
Avoid this by only calling the function if  hard_iface not null.

Detected by CoverityScan, CID#1466456 ("Explicit null dereferenced")

Fixes: 53dd9a68 ("batman-adv: add multicast flags netlink support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>