[core] fix segfault if tempdirs fill up (fixes #2843)

(thx wolfram)

x-ref:
  "lighttpd segfault if /var/tmp is full"
  https://redmine.lighttpd.net/issues/2843

src/chunk.c

@@ -741,7 +741,7 @@ static void chunkqueue_remove_empty_chunks(chunkqueue *cq) {
 	chunkqueue_remove_finished_chunks(cq);
 	if (chunkqueue_is_empty(cq)) return;
 
-	for (c = cq->first; c->next; c = c->next) {
+	for (c = cq->first; c && c->next; c = c->next) {
 		if (0 == chunk_remaining_length(c->next)) {
 			chunk *empty = c->next;
 			c->next = empty->next;