avcodec/audiotoolboxenc: return external error if encode failed

because the AudioConverterFillComplexBuffer can return 0 or 1 if
success.
so set the ret to 0 it AudioConverterFillComplexBuffer success and
return ret value for success or return AVERROR_EXTERNAL when
AudioConverterFillComplexBuffer failed.
BTW change the error message log level from warning to error.

Reviewed-by: Zhao Zhili <zhilizhao@tencent.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavcodec/audiotoolboxenc.c

@@ -554,11 +554,13 @@ static int ffat_encode(AVCodecContext *avctx, AVPacket *avpkt,
                                      avctx->frame_size,
                            &avpkt->pts,
                            &avpkt->duration);
+        ret = 0;
     } else if (ret && ret != 1) {
-        av_log(avctx, AV_LOG_WARNING, "Encode error: %i\n", ret);
+        av_log(avctx, AV_LOG_ERROR, "Encode error: %i\n", ret);
+        ret = AVERROR_EXTERNAL;
     }
 
-    return 0;
+    return ret;
 }
 
 static av_cold void ffat_encode_flush(AVCodecContext *avctx)