avfilter/f_loop: fix length of aloop leftover buffer

If the audio loop stops inside an audio frame, the leftover buffer contains the
end of the frame, which is not looped. The length supposed to be the part which
was not written to the loop buffer, so we need to drain exactly that number of
bytes from the leftover buffer.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit b33a594160)

libavfilter/f_loop.c

@@ -169,14 +169,13 @@ static int afilter_frame(AVFilterLink *inlink, AVFrame *frame)
                 s->pts += av_rescale_q(s->start - s->ignored_samples, (AVRational){1, outlink->sample_rate}, outlink->time_base);
             }
             s->nb_samples += ret - drain;
-            drain = frame->nb_samples - written;
-            if (s->nb_samples == s->size && drain > 0) {
+            if (s->nb_samples == s->size && frame->nb_samples > written) {
                 int ret2;
 
                 ret2 = av_audio_fifo_write(s->left, (void **)frame->extended_data, frame->nb_samples);
                 if (ret2 < 0)
                    return ret2;
-                av_audio_fifo_drain(s->left, drain);
+                av_audio_fifo_drain(s->left, written);
             }
             frame->nb_samples = ret;
             s->pts += av_rescale_q(ret, (AVRational){1, outlink->sample_rate}, outlink->time_base);