Bugfix: Don't check the genesis block header before accepting it

This fixes an error triggered when running with -reindex after #5975

src/main.cpp

@@ -2809,6 +2809,8 @@ bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBloc
     uint256 hash = block.GetHash();
     BlockMap::iterator miSelf = mapBlockIndex.find(hash);
     CBlockIndex *pindex = NULL;
+    if (hash != chainparams.GetConsensus().hashGenesisBlock) {
+
         if (miSelf != mapBlockIndex.end()) {
             // Block header is already known.
             pindex = miSelf->second;
@@ -2824,21 +2826,20 @@ bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, CBloc
 
         // Get prev block index
         CBlockIndex* pindexPrev = NULL;
-    if (hash != chainparams.GetConsensus().hashGenesisBlock) {
         BlockMap::iterator mi = mapBlockIndex.find(block.hashPrevBlock);
         if (mi == mapBlockIndex.end())
             return state.DoS(10, error("%s: prev block not found", __func__), 0, "bad-prevblk");
         pindexPrev = (*mi).second;
         if (pindexPrev->nStatus & BLOCK_FAILED_MASK)
             return state.DoS(100, error("%s: prev block invalid", __func__), REJECT_INVALID, "bad-prevblk");
-    }
+
         assert(pindexPrev);
         if (fCheckpointsEnabled && !CheckIndexAgainstCheckpoint(pindexPrev, state, chainparams, hash))
             return error("%s: CheckIndexAgainstCheckpoint(): %s", __func__, state.GetRejectReason().c_str());
 
         if (!ContextualCheckBlockHeader(block, state, pindexPrev))
             return false;
-
+    }
     if (pindex == NULL)
         pindex = AddToBlockIndex(block);