Fix read-after-eof panic (#6)

Fixed a panic that was caused when Read() was called after a previous Read() returned io.EOF

xfs/xfs.go

@@ -458,6 +458,9 @@ func (f *File) Stat() (fs.FileInfo, error) {
 }
 
 func (f *File) Read(buf []byte) (int, error) {
+	if f.buffer == nil {
+		return 0, io.EOF
+	}
 	if f.buffer.Len() == 0 {
 		f.currentBlock++
 		if f.currentBlock*f.blockSize >= f.Size() {