batman-adv: Ignore neighbor throughput metrics in error case

commit e7e34ffc976aaae4f465b7898303241b81ceefc3 upstream.

If a temporary error happened in the evaluation of the neighbor throughput
information, then the invalid throughput result should not be stored in the
throughtput EWMA.

Cc: stable@vger.kernel.org
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>