usb: hub: PDO: New checking in IsValidPDO()

As child's parent FDO at any moment can receive surprise removal,
Before being it should notify it's childs by setting their parents to NULL.
So if we see PDO with parent==NULL then we can consider it as physically
removed, i.e. invalid.

svn path=/branches/GSoC_2016/USB/; revision=72368

Author: mvardan

drivers/usb/usbhub/pdo.c

@@ -146,6 +146,11 @@ IsValidPDO(
 
     ChildDeviceExtension = (PHUB_CHILDDEVICE_EXTENSION)DeviceObject->DeviceExtension;
     ASSERT(ChildDeviceExtension->Common.IsFDO == FALSE);
+
+    // This can happen when parent device was surprise removed.
+    if (ChildDeviceExtension->ParentDeviceObject == NULL)
+        return FALSE;
+
     HubDeviceExtension = (PHUB_DEVICE_EXTENSION)ChildDeviceExtension->ParentDeviceObject->DeviceExtension;
 
     for(Index = 0; Index < USB_MAXCHILDREN; Index++)