seeking: Favor pkt->pos for generic index building where possible.

See [FFmpeg-devel] [PATCH] Only using st->parser->pos when doing?repacking in the parser.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -1102,8 +1102,9 @@ static int av_read_frame_internal(AVFormatContext *s, AVPacket *pkt)
                     compute_pkt_fields(s, st, st->parser, pkt);
 
                     if((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY){
+                        int64_t pos= (st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) ? pkt->pos : st->parser->frame_offset;
                         ff_reduce_index(s, st->index);
-                        av_add_index_entry(st, st->parser->frame_offset, pkt->dts,
+                        av_add_index_entry(st, pos, pkt->dts,
                                            0, 0, AVINDEX_KEYFRAME);
                     }