lavu/opt: check if class is NULL too

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavutil/opt.c

@@ -63,7 +63,7 @@ const AVOption *av_next_option(void *obj, const AVOption *last)
 const AVOption *av_opt_next(void *obj, const AVOption *last)
 {
     AVClass *class = *(AVClass**)obj;
-    if (!last && class->option && class->option[0].name)
+    if (!last && class && class->option && class->option[0].name)
         return class->option;
     if (last && last[1].name)
         return ++last;