ac3enc_template: Use the correct context field

For audio encoders, delay has no effect, use the appropriate one,
initial_padding (see 2df0c32e).

libavcodec/ac3enc_template.c

@@ -450,7 +450,7 @@ int AC3_NAME(encode_frame)(AVCodecContext *avctx, AVPacket *avpkt,
     ff_ac3_output_frame(s, avpkt->data);
 
     if (frame->pts != AV_NOPTS_VALUE)
-        avpkt->pts = frame->pts - ff_samples_to_time_base(avctx, avctx->delay);
+        avpkt->pts = frame->pts - ff_samples_to_time_base(avctx, avctx->initial_padding);
 
     *got_packet_ptr = 1;
     return 0;