opt: avoid segfault in av_opt_next() if the class does not have an option list

CC: libav-stable@libav.org

libavutil/opt.c

@@ -37,8 +37,10 @@
 const AVOption *av_opt_next(void *obj, const AVOption *last)
 {
     AVClass *class = *(AVClass**)obj;
-    if (!last && class->option[0].name) return class->option;
-    if (last && last[1].name)           return ++last;
+    if (!last && class->option && class->option[0].name)
+        return class->option;
+    if (last && last[1].name)
+        return ++last;
     return NULL;
 }