ffplay: use start_time as next audio pts on flush when seeking is not supported

Theoretically using start_time should also work if seeking is available and we
could determine that the next packet after a flush packet is the first packet
of a stream, but I could not think of an easy and clean way to do that, that is
why I sticked to the no seeking available condition for now.

Fixes ticket #2647.

Signed-off-by: Marton Balint <cus@passwd.hu>