[Consensus] Guard time checks changes

Guard these changes against the protocol version change enforcement

Author: Warrows

src/kernel.cpp

@@ -431,7 +431,7 @@ bool CheckProofOfStake(const CBlock block, uint256& hashProofOfStake, std::uniqu
 
     unsigned int nBlockFromTime = blockfrom.nTime;
     unsigned int nTxTime = block.nTime;
-    if (!txin.IsZerocoinSpend()) { //Equivalent for zPIV is checked above in ContextualCheckZerocoinStake()
+    if (!txin.IsZerocoinSpend() && nPreviousBlockHeight >= Params().Zerocoin_Block_Public_Spend_Enabled() - 1) { //Equivalent for zPIV is checked above in ContextualCheckZerocoinStake()
         if (nTxTime < nBlockFromTime) // Transaction timestamp nTxTime
             return error("CheckStakeKernelHash() : nTime violation - nBlockFromTime=%d nTimeTx=%d", nBlockFromTime, nTxTime);
         if (nBlockFromTime + nStakeMinAge > nTxTime) // Min age requirement