cache->idset can be NULL as I just discovered.  This is more for
nitpicking' sake rather than anything, but it avoids a crash when
debugging the object cache and hitting the limits.

diff /home/op/w/got
commit - 98275f2eefb932aee3f1824f53c268fd736f6c5f
path + /home/op/w/got
blob - 9a73324894a8116700bddcfd4f59c9a3a39dd2e5
file + lib/object_cache.c
--- lib/object_cache.c
+++ lib/object_cache.c
@@ -294,7 +294,7 @@ print_cache_stats(struct got_object_cache *cache, cons
 {
 	fprintf(stderr, "%s: %s cache: %d elements, %d searches, %d hits, "
 	    "%d missed, %d evicted, %d too large\n", getprogname(), name,
-	    got_object_idset_num_elements(cache->idset),
+	    cache->idset ? got_object_idset_num_elements(cache->idset) : -1,
 	    cache->cache_searches, cache->cache_hit,
 	    cache->cache_miss, cache->cache_evict, cache->cache_toolarge);
 }
@@ -378,7 +378,8 @@ got_object_cache_close(struct got_object_cache *cache)
 		break;
 	}
 
-	got_object_idset_for_each(cache->idset, check_refcount, cache);
+	if (cache->idset)
+		got_object_idset_for_each(cache->idset, check_refcount, cache);
 #endif
 
 	if (cache->idset) {