rtsp: safer buffer handling

rtsp.c

@@ -406,7 +406,7 @@ static void handle_announce(rtsp_conn_info *conn,
  char *cp = req->content;
  int cp_left = req->contentlength;
  char *next;
- while (cp) {
+ while (cp_left && cp) {
  next = nextline(cp, cp_left);
  cp_left -= next-cp;