ffmpeg: use pkt_pts before pkt->pts for audio

This fixes the timestamp of the first packet when samples have been skiped.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2383,7 +2383,10 @@ static int decode_audio(InputStream *ist, AVPacket *pkt, int *got_output)
        the decoder could be delaying output by a packet or more. */
     if (decoded_frame->pts != AV_NOPTS_VALUE)
         ist->dts = ist->next_dts = ist->pts = ist->next_pts = decoded_frame->pts;
-    else if (pkt->pts != AV_NOPTS_VALUE) {
+    else if (decoded_frame->pkt_pts != AV_NOPTS_VALUE) {
+        decoded_frame->pts = decoded_frame->pkt_pts;
+        pkt->pts           = AV_NOPTS_VALUE;
+    } else if (pkt->pts != AV_NOPTS_VALUE) {
         decoded_frame->pts = pkt->pts;
         pkt->pts           = AV_NOPTS_VALUE;
     }else