net: Check if nc is NULL in qemu_get_vnet_hdr_len()

net: Check if nc is NULL in qemu_get_vnet_hdr_len()

A netdev may not have a peer specified, resulting in NULL. We should
make it behave like /dev/null in such a case instead of letting it
cause segmentatin fault.

Fixes: 4b52d63249a5 ("tap: Remove qemu_using_vnet_hdr()")
Cc: qemu-stable@nongnu.org
Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Tested-by; Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>