vsock: cope with memory allocation failure at socket creation time

In the unlikely event that the kmalloc call in vmci_transport_socket_init()
fails, we end-up calling vmci_transport_destruct() with a NULL vmci_trans()
and oopsing.

This change addresses the above explicitly checking for zero vmci_trans()
at destruction time.

Reported-by: Xiumei Mu <xmu@redhat.com>
Fixes: d021c344

 ("VSOCK: Introduce VM Sockets")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>