lavfi/buffersink: loop over ff_request_frame().

Do not assume that ff_request_frame() returning success
implies a frame has arrived in the FIFO.
Instead, just loop until a frame is in the FIFO.
It does not change anything since the same loop is present
in ff_request_frame(), confirmed by an assertion.

libavfilter/buffersink.c

@@ -133,7 +133,7 @@ int attribute_align_arg av_buffersink_get_frame_flags(AVFilterContext *ctx, AVFr
     AVFrame *cur_frame;
 
     /* no picref available, fetch it from the filterchain */
-    if (!av_fifo_size(buf->fifo)) {
+    while (!av_fifo_size(buf->fifo)) {
         if (inlink->closed)
             return AVERROR_EOF;
         if (flags & AV_BUFFERSINK_FLAG_NO_REQUEST)
@@ -142,9 +142,6 @@ int attribute_align_arg av_buffersink_get_frame_flags(AVFilterContext *ctx, AVFr
             return ret;
     }
 
-    if (!av_fifo_size(buf->fifo))
-        return AVERROR(EINVAL);
-
     if (flags & AV_BUFFERSINK_FLAG_PEEK) {
         cur_frame = *((AVFrame **)av_fifo_peek2(buf->fifo, 0));
         if ((ret = av_frame_ref(frame, cur_frame)) < 0)