xen/pciback: Check if the device is found instead of blindly assuming so.

Just in case it is not found, don't try to dereference it.

[v1: Added WARN_ON, suggested by Jan Beulich <JBeulich@suse.com>]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xen-pciback/pci_stub.c

@@ -220,6 +220,8 @@ void pcistub_put_pci_dev(struct pci_dev *dev)
 	}
 
 	spin_unlock_irqrestore(&pcistub_devices_lock, flags);
+	if (WARN_ON(!found_psdev))
+		return;
 
 	/*hold this lock for avoiding breaking link between
 	* pcistub and xen_pcibk when AER is in processing