ac3dec: fix non-optimal dithering of zero bit mantissas

Use a noise range of -0.707 to 0.707 instead of -0.5 to 0.5

Based on patch by: Mathias Rauen <madshi@gmail.com> and commit by Justin Ruggles (04ea5491)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>