diracdec: avoid overflow of bytes*8 in decode_lowdelay

If bytes is large enough, bytes*8 can overflow and become negative.

In that case 'bufsize -= bytes*8' causes bufsize to increase instead of
decrease.

This leads to a segmentation fault.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
