message: cast nTime to int64_t
  to avoid an unsigned result then it will be sign extended
diff: @@ -296,7 +296,7 @@ static arith_uint256 ComputeTarget(const CBlockIndex *pindexFirst,
  // In order to avoid difficulty cliffs, we bound the amplitude of the
  // adjustement we are going to do.
  - int64_t nActualTimespan = pindexLast->nTime - pindexFirst->nTime;
  + int64_t nActualTimespan = int64_t(pindexLast->nTime) - int64_t(pindexFirst->nTime);
  if (nActualTimespan > 288 * params.nPowTargetSpacing)
  {
  nActualTimespan = 288 * params.nPowTargetSpacing;
  