[KERNEL32] GetDriveTypeW: Fix the case with unformatted volumes and volume GUID names

Fix the case when GetDriveTypeW would return DRIVE_NO_ROOT_DIR when it
got a volume GUID name and had to access an unformatted volume.
Fixes Rufus not being able to find an unformatted volume after writting new MBR.

dll/win32/kernel32/client/file/disk.c

···
         return DRIVE_NO_ROOT_DIR;
     }
 
+    /* We will work with a device object, so trim the trailing backslash now */
+    PathName.Length -= sizeof(WCHAR);
+
     /* Let's probe for it, by forcing open failure! */
     RetryOpen = TRUE;
     InitializeObjectAttributes(&ObjectAttributes, &PathName,