diracdec: prevent overflow in data_unit_size check

buf_idx + data_unit_size can overflow, causing the '> buf_size' check to
wrongly fail.

This causes a segmentation fault.

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