lavfi/alphamerge: raise filter_frame() error.

libavfilter/vf_alphamerge.c

@@ -131,12 +131,13 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
     AVFilterContext *ctx = inlink->dst;
     AlphaMergeContext *merge = ctx->priv;
 
+    int ret = 0;
     int is_alpha = (inlink == ctx->inputs[1]);
     struct FFBufQueue *queue =
         (is_alpha ? &merge->queue_alpha : &merge->queue_main);
     ff_bufqueue_add(ctx, queue, buf);
 
-    while (1) {
+    do {
         AVFrame *main_buf, *alpha_buf;
 
         if (!ff_bufqueue_peek(&merge->queue_main, 0) ||
@@ -147,10 +148,10 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *buf)
 
         merge->frame_requested = 0;
         draw_frame(ctx, main_buf, alpha_buf);
-        ff_filter_frame(ctx->outputs[0], main_buf);
+        ret = ff_filter_frame(ctx->outputs[0], main_buf);
         av_frame_free(&alpha_buf);
-    }
-    return 0;
+    } while (ret >= 0);
+    return ret;
 }
 
 static int request_frame(AVFilterLink *outlink)