amr: Set the pkt->pos field properly to the start of the packet

Previously, the field pointed to the second byte of the packet
(which is the first byte of the actual AMR payload).

libavformat/amr.c

@@ -121,6 +121,7 @@ static int amr_read_packet(AVFormatContext *s,
 {
     AVCodecContext *enc = s->streams[0]->codec;
     int read, size = 0, toc, mode;
+    int64_t pos = avio_tell(s->pb);
 
     if (s->pb->eof_reached)
     {
@@ -157,7 +158,7 @@ static int amr_read_packet(AVFormatContext *s,
     s->streams[0]->codec->bit_rate = size*8*50;
 
     pkt->stream_index = 0;
-    pkt->pos= avio_tell(s->pb);
+    pkt->pos = pos;
     pkt->data[0]=toc;
     pkt->duration= enc->codec_id == CODEC_ID_AMR_NB ? 160 : 320;
     read = avio_read(s->pb, pkt->data+1, size-1);