squashfs: set file inode's frag index when no fragment data

Due to spec, frag index of file inode

https://dr-emann.github.io/squashfs/squashfs.html#_file_inodes

```
An index into the Fragment Table which describes the fragment block
that the tail end of this file is stored in. If not used, this is set
to 0xFFFFFFFF.
```

Signed-off-by: Xynnn007 <[email protected]>

Author: Xynnn007

filesystem/squashfs/finalize.go

@@ -1128,6 +1128,8 @@ func createInodes(fileList []*finalizeFileInfo, idtable map[uint32]uint16, optio
 				if e.fragment != nil {
 					bf.fragmentBlockIndex = e.fragment.block
 					bf.fragmentOffset = e.fragment.offset
+				} else {
+					bf.fragmentBlockIndex = 0xffffffff
 				}
 				in = bf
 				inodeT = inodeBasicFile