Use avio_get_dyn_buf()+ffio_free_dyn_buf() instead of
avio_close_dyn_buf()+av_free(). This saves an allocation
(and memcpy) in case all the data fits in the AVIOContext's
write buffer.
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>