Use block height 0 when checking for genesis block

There is no previous block when validating the genesis block as part of contextual checks,
so block height cannot be used. Consensus parameters are now taken from height 0 for
the genesis check, and from block height only if the it's not the genesis block.

src/main.cpp

@@ -2747,14 +2747,14 @@ bool CheckBlock(const CBlock& block, CValidationState& state, bool fCheckPOW, bo
 bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& state, CBlockIndex * const pindexPrev)
 {
     const CChainParams& chainParams = Params();
-    const Consensus::Params& consensusParams = chainParams.GetConsensus(pindexPrev->nHeight + 1);
     uint256 hash = block.GetHash();
-    if (hash == consensusParams.hashGenesisBlock)
+    if (hash == chainParams.GetConsensus(0).hashGenesisBlock)
         return true;
 
     assert(pindexPrev);
 
     int nHeight = pindexPrev->nHeight+1;
+    const Consensus::Params& consensusParams = chainParams.GetConsensus(nHeight);
 
     // Disallow legacy blocks after merge-mining start.
     if (!consensusParams.fAllowLegacyBlocks