vhost_net: validate sock before trying to put its fd

Sock will be NULL if we pass -1 to vhost_net_set_backend(), but when
we meet errors during ubuf allocation, the code does not check for
NULL before calling sockfd_put(), this will lead NULL
dereferencing. Fixing by checking sock pointer before.

Fixes: bab632d6 ("vhost: vhost TX zero-copy support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>