lavfi/buffersink: allow av_buffersink_set_frame_size() to be called earlier

The function currently writes directly into the input link, which
requires it to be called after the filter has been linked. However, the
documentation does not mention this restriction and the natural place to
call av_buffersink_set_frame_size() during the options-setting stage,
that is before filter init (and so before the input link exists).

libavfilter/buffersink.c

@@ -42,6 +42,7 @@
 typedef struct BufferSinkContext {
     const AVClass *class;
     unsigned warning_limit;
+    unsigned frame_size;
 
     /* only used for video */
     enum AVPixelFormat *pixel_fmts;     ///< list of accepted pixel formats
@@ -162,11 +163,25 @@ static int activate(AVFilterContext *ctx)
     return 0;
 }
 
+static int config_input_audio(AVFilterLink *inlink)
+{
+    BufferSinkContext *buf = inlink->dst->priv;
+    FilterLink *l = ff_filter_link(inlink);
+
+    l->min_samples = l->max_samples = buf->frame_size;
+
+    return 0;
+}
+
 void av_buffersink_set_frame_size(AVFilterContext *ctx, unsigned frame_size)
 {
-    FilterLink *inlink = ff_filter_link(ctx->inputs[0]);
+    BufferSinkContext *buf = ctx->priv;
+    buf->frame_size = frame_size;
 
-    inlink->min_samples = inlink->max_samples = frame_size;
+    if (ctx->inputs && ctx->inputs[0]) {
+        FilterLink *l = ff_filter_link(ctx->inputs[0]);
+        l->min_samples = l->max_samples = buf->frame_size;
+    }
 }
 
 #define MAKE_AVFILTERLINK_ACCESSOR(type, field) \
@@ -368,6 +383,14 @@ const AVFilter ff_vsink_buffer = {
     FILTER_QUERY_FUNC(vsink_query_formats),
 };
 
+static const AVFilterPad inputs_audio[] = {
+    {
+        .name         = "default",
+        .type         = AVMEDIA_TYPE_AUDIO,
+        .config_props = config_input_audio,
+    },
+};
+
 const AVFilter ff_asink_abuffer = {
     .name          = "abuffersink",
     .description   = NULL_IF_CONFIG_SMALL("Buffer audio frames, and make them available to the end of the filter graph."),
@@ -376,7 +399,7 @@ const AVFilter ff_asink_abuffer = {
     .init          = common_init,
     .uninit        = uninit,
     .activate      = activate,
-    FILTER_INPUTS(ff_audio_default_filterpad),
+    FILTER_INPUTS(inputs_audio),
     .outputs       = NULL,
     FILTER_QUERY_FUNC(asink_query_formats),
 };