Adjust consensus rule to accept genesis block without height in coinbase

Closes #1753

src/main.cpp

@@ -3090,7 +3090,10 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
 
     // Enforce block.nVersion=2 rule that the coinbase starts with serialized block height
     // if 750 of the last 1,000 blocks are version 2 or greater (51/100 if testnet):
-    if (block.nVersion >= 2)
+    // Since MIN_BLOCK_VERSION = 4 all blocks with nHeight > 0 should satisfy this.
+    // This rule is not applied to the genesis block, which didn't include the height
+    // in the coinbase.
+    if (nHeight > 0)
     {
         CScript expect = CScript() << nHeight;
         if (block.vtx[0].vin[0].scriptSig.size() < expect.size() ||