macvlan: Fix leak and NULL dereference on error path

The recent patch that moved broadcasts to process context added
a couple of bugs on the error path where we may dereference NULL
or leak an skb.  This patch fixes them.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>