examples/decode_filter_audio: Add loop for draining the filtergraph

Depending on the filters used, the filtergraph may produce trailing data
after feeding it the last input frame. Update the example to include the
necessary loop for draining the filtergraph.

Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Tobias Rapp <t.rapp@noa-archive.com>

doc/examples/decode_filter_audio.c

@@ -279,6 +279,25 @@ int main(int argc, char **argv)
         }
         av_packet_unref(packet);
     }
+    if (ret == AVERROR_EOF) {
+        /* signal EOF to the filtergraph */
+        if (av_buffersrc_add_frame_flags(buffersrc_ctx, NULL, 0) < 0) {
+            av_log(NULL, AV_LOG_ERROR, "Error while closing the filtergraph\n");
+            goto end;
+        }
+
+        /* pull remaining frames from the filtergraph */
+        while (1) {
+            ret = av_buffersink_get_frame(buffersink_ctx, filt_frame);
+            if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
+                break;
+            if (ret < 0)
+                goto end;
+            print_frame(filt_frame);
+            av_frame_unref(filt_frame);
+        }
+    }
+
 end:
     avfilter_graph_free(&filter_graph);
     avcodec_free_context(&dec_ctx);