Closed tiny memory leak detected by valgrind.

libisofs/fs_image.c

@@ -1601,6 +1601,7 @@ int iso_file_source_new_ifs(IsoImageFilesystem *fs, IsoFileSource *parent,
                            cs_value);
                 }
                 iso_msgs_submit(0, msg, 0, "NOTE", 0);
+                free(cs_value);
                 cs_value = NULL;
             }
         }