sfemem_iterator: Can have segments to deref even if next segment not read ahead

The next segment might not be available yet when we try to
read ahead.

src/fellow_cache.c

@@ -3097,9 +3097,6 @@ fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
 	flush = 0;
 	while ((fcs = fcsc_next(&c)) != NULL) {
 		if (ra[n % mod] == NULL) {
-			/* We always flush for readahead == 0, see below for
-			 * explanation */
-			AZ(derefn);
 			fellow_cache_seg_ref_in(fc, FCIO_SYNC, &fcs, 1);
 			ra[n % mod] = fcs;
 		}