avformat/dvenc: Replace write_trailer by deinit function

avformat/dvenc: Replace write_trailer by deinit function

The old write_trailer only freed memory, so it is better to make a
dedicated deinit function out of it. Given that this function will also
be called when writing the header fails, one can also remove code that
frees already allocated fifos when allocating another one fails.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>