New API call iso_image_generator_is_running().

Prevented a potential race condition between Ecma119Image disposal by
burn_source and final activities of ISO generator thread.

libisofs/image.h

@@ -166,6 +166,12 @@ struct Iso_Image
     uint32_t checksum_idx_count;
     char *checksum_array;
 
+    /**
+     * Whether a write run has been started by iso_image_create_burn_source()
+     * and has not yet been finished.
+     */
+    int generator_is_running;
+
 };