lavu/opt: handle NULL obj in av_opt_next

It indirectly also fixes av_opt_free for NULL objs.
Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>

libavutil/opt.c

@@ -50,7 +50,10 @@ const AVOption *av_next_option(void *obj, const AVOption *last)
 
 const AVOption *av_opt_next(void *obj, const AVOption *last)
 {
-    AVClass *class = *(AVClass**)obj;
+    const AVClass *class;
+    if (!obj)
+        return NULL;
+    class = *(const AVClass**)obj;
     if (!last && class && class->option && class->option[0].name)
         return class->option;
     if (last && last[1].name)