block: xen-blkfront: Fix possible NULL ptr dereference

In the blkif_release function the bdget_disk() call might returns
a NULL ptr which might be dereferenced on bdev->bd_openers checking

Signed-off-by: Felipe Pena <felipensp@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[v2: Added WARN per Roger's suggestion]