LogicSegment::pow2_ceil(): Fix potentіal integer overflow.

Reported by Coverity (CID 50925):

  Unintentional integer overflow (OVERFLOW_BEFORE_WIDEN)
    overflow_before_widen: Potentially overflowing expression 1 << power
    with type int (32 bits, signed) is evaluated using 32-bit arithmetic,
    and then used in a context that expects an expression of type
    uint64_t const (64 bits, unsigned).