hvc_xen: NULL dereference on allocation failure

commit 201a52bea928687b7557728b176ac4f8a37d5cbd upstream.

If kzalloc() returns a NULL here, we pass a NULL to
xencons_disconnect_backend() which will cause an Oops.

Also I removed the __GFP_ZERO while I was at it since kzalloc() implies
__GFP_ZERO.

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>