AVOptions: don't segfault on NULL parameter in av_set_options_string()

libavutil/opt.c

@@ -696,6 +696,9 @@ int av_set_options_string(void *ctx, const char *opts,
 {
     int ret, count = 0;
 
+    if (!opts)
+        return 0;
+
     while (*opts) {
         if ((ret = parse_key_value_pair(ctx, &opts, key_val_sep, pairs_sep)) < 0)
             return ret;