avformat/ttaenc: Defer freeing dynamic buffer

avformat/ttaenc: Defer freeing dynamic buffer

The TTA muxer writes a seektable in a dynamic buffer as it receives
packets and when writing the trailer, closes the dynamic buffer using
avio_close_dyn_buf(), writes the seektable and frees the buffer. But
the TTA muxer already has a deinit function which unconditionally
calls ffio_free_dyn_buf() on the dynamic buffer, so switching to
avio_get_dyn_buf() means that one can remove the code to free the
buffer; furthermore, it also might save an allocation if the seektable
is so small that it fits into the dynamic buffer's write buffer or if
adding the padding that avio_close_dyn_buf() adds necessitated
reallocating of the underlying buffer.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>