fix void pointer arithmetic

src/write.c

@@ -324,7 +324,7 @@ enum ec3_status ec3_tag_writer_write(
 	size_t cluster_size
 		= cluster_sizes[w->w_parent->w_param.p_cluster_size];
 	enum ec3_status status = EC3_SUCCESS;
-	void *buf = w->w_buf;
+	unsigned char *buf = w->w_buf;
 
 	while (len > 0) {
 		size_t remaining = cluster_size - w->w_ptr;