bridge: Fix skb leak when multicast parsing fails on TX

On the bridge TX path we're leaking an skb when br_multicast_rcv
returns an error.

Reported-by: David Lamparter <equinox@diac24.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>