batman-adv: Return directly after a failed batadv_dat_select_candidates() in...

batman-adv: Return directly after a failed batadv_dat_select_candidates() in batadv_dat_forward_data()

The kfree() function was called in one case by
the batadv_dat_forward_data() function during error handling
even if the passed variable contained a null pointer.
This issue was detected by using the Coccinelle software.

* Thus return directly after a batadv_dat_select_candidates() call failed
  at the beginning.

* Delete the label “out” which became unnecessary with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>