The storage element may be empty on fetch fail

A failed fetch won't have the storage trimmed, which may result in an
empty storage element in the body list. Do not assert on non-empty
size in the simple iterator.

Fixes: #2186

bin/varnishd/storage/storage_simple.c

@@ -260,8 +260,7 @@ sml_iterator(struct worker *wrk, struct objcore *oc,
 
 	if (boc == NULL) {
 		VTAILQ_FOREACH_SAFE(st, &obj->list, list, checkpoint) {
-			AN(st->len);
-			if (ret == 0)
+			if (ret == 0 && st->len > 0)
 				ret = func(priv, 1, st->ptr, st->len);
 			if (final) {
 				VTAILQ_REMOVE(&obj->list, st, list);