Also use a unsigned constant for the shift calculation, as 1 << 31 is
undefined for int32_t. This is also fixed oggparsetheora.

This fixes ubsan runtime error: shift exponent is too large for
32-bit type 'int'
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>