avcodec/fate-idct8x8: Defined behavior: eliminate negative left-shifts.

The original code left-shifts negative values, which is undefined
in the C99 specification (the one used during normal FFmpeg compilation).
This change multiplies by (1 << shift), which is functionally equivalent,
but has defined behavior.
With this change, fate-idct8x8 compiled with --fsanitize=undefined works.

Bug-Id: 686

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

The assembly generated by gcc is unchanged by this commit
