bridge: Fix OOM crash in deliver_clone

The bridge multicast patches introduced an OOM crash in the forward
path, when deliver_clone fails to clone the skb.

Reported-by: Mark Wagner <mwagner@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>