avformat/mux: Restore original ts in write_packet on error

Restore original timestamps in write_packet() if the
actual write operation was not successfull. This allows
to pass the same packet to nonblocking muxer repeatedly
without corrupting the timestamps.

Signed-off-by: Jan Sebechlebsky <sebechlebskyjan@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
