avcodec: Fix duration error after seek operation

After seek, s->duration need recalculation, AVCodecParserContext need be set
again. Without the fix, the first audio packet would be dropped after
compute_pkt_fields() as the final pts/dts become invalid.

Signed-off-by: wangxingchao <wangxingchao@xiaomi.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>