lavfi/aiir: Refine the pad/vpad related operation

move the pad/vpad related operation with more natural
coding style.
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/af_aiir.c

@@ -1180,6 +1180,10 @@ static av_cold int init(AVFilterContext *ctx)
     if (!pad.name)
         return AVERROR(ENOMEM);
 
+    ret = ff_insert_outpad(ctx, 0, &pad);
+    if (ret < 0)
+        return ret;
+
     if (s->response) {
         vpad = (AVFilterPad){
             .name         = av_strdup("filter_response"),
@@ -1188,13 +1192,7 @@ static av_cold int init(AVFilterContext *ctx)
         };
         if (!vpad.name)
             return AVERROR(ENOMEM);
-    }
 
-    ret = ff_insert_outpad(ctx, 0, &pad);
-    if (ret < 0)
-        return ret;
-
-    if (s->response) {
         ret = ff_insert_outpad(ctx, 1, &vpad);
         if (ret < 0)
             return ret;