Added more necessary parts to compression.

writer_compress.go

@@ -0,0 +1,17 @@
+package squashfs
+
+import "reflect"
+
+func (w *Writer) compressData(data []byte) ([]byte, error) {
+	if reflect.DeepEqual(data, make([]byte, len(data))) {
+		return nil, nil
+	}
+	compressedData, err := w.compressor.Compress(data)
+	if err != nil {
+		return nil, err
+	}
+	if len(data) <= len(compressedData) {
+		return data, nil
+	}
+	return compressedData, nil
+}