message: validation: remove redundant check on pindex
  This removes a conditional that checks if pindex is equal to nullptr.
  This check is redundant because the branch where pindex is set returns at an earlier time. Additionaly, The independence of the earlier and later pindex is made clearer.
diff: @@ -3415,11 +3415,10 @@ bool BlockManager::AcceptBlockHeader(const CBlockHeader& block, BlockValidationS
  // Check for duplicate
  uint256 hash = block.GetHash();
  BlockMap::iterator miSelf = m_block_index.find(hash);
  - CBlockIndex *pindex = nullptr;
  if (hash != chainparams.GetConsensus().hashGenesisBlock) {
  if (miSelf != m_block_index.end()) {
  // Block header is already known.
  - pindex = miSelf->second;
  + CBlockIndex* pindex = miSelf->second;
  if (ppindex)
  *ppindex = pindex;
  if (pindex->nStatus & BLOCK_FAILED_MASK) {
  @@ -3488,8 +3487,7 @@ bool BlockManager::AcceptBlockHeader(const CBlockHeader& block, BlockValidationS
  }
  }
  }
  - if (pindex == nullptr)
  - pindex = AddToBlockIndex(block);
  + CBlockIndex* pindex = AddToBlockIndex(block);
  if (ppindex)
  *ppindex = pindex;
  