lavd/sdl: reset context variables after destroy

SDL device reports double free or hangs when quiting.
Reseting variables solves the issue.

The issue is caused by double call of write_trailer callback.
It is called for the first time in write_packet when quit flag is set.
Second call comes from ffmpeg.c or may come from other client application.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
