casereader: Make casereader_random_class function 'advance' optional.

[pspp] / src / data / casereader.c

diff --git a/src/data/casereader.c b/src/data/casereader.c
index 1b06f42e8ab399b9b0ad4a3b76f1a3ea82a10bea..a410afcbcbfab2c826735d9ef94e4daffdca10f5 100644 (file)
--- a/src/data/casereader.c
+++ b/src/data/casereader.c
@@ -501,7 +501,8 @@ advance_random_reader (struct casereader *reader,
   if (new > old)
     {
       shared->min_offset = new;
-      shared->class->advance (reader, shared->aux, new - old);
+      if (shared->class->advance)
+        shared->class->advance (reader, shared->aux, new - old);
     }
 }