If truehd_core strips Atmos data away, three parts of the output differ
in size compared to the input access unit: a) The major_sync_info block
if the extra_channel_meaning_data is present, as the newly written
output never contains said block; b) the substream_directory (because
entries relating to discarded substreams are discarded, too); and c)
the actual substream data. b) and c) have already been taken into account
when choosing the size of the output packet, but a) has been forgotten.

This is also the reason behind the end of the output buffer having been
uninitialized until 801d78f0. The workaround added in said commit has
been removed, too.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>