lavfi/buffersrc: fix failed requests count.

The line was lost during the AVFrame transition.
It causes programs relying on the failed requests count,
especially ffmpeg, to add frames to the wrong input.

Fix trac ticket #2467.

libavfilter/buffersrc.c

@@ -123,6 +123,8 @@ static int attribute_align_arg av_buffersrc_add_frame_internal(AVFilterContext *
     AVFrame *copy;
     int ret;
 
+    s->nb_failed_requests = 0;
+
     if (!frame) {
         s->eof = 1;
         return 0;