aacsbr: silence message for SBR extension "padding".

Some files contain a few additional, all-0 bits.
Check for that case and don't print incorrect "not supported"
message.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Alex Converse <alex.converse@gmail.com>
