Merge bitcoin/bitcoin#28791: snapshots: don't core dump when running -checkblockindex after loadtxoutset

cdc6ac4 snapshots: don't core dump when running -checkblockindex after `loadtxoutset` (Mark Friedenbach)

Pull request description:

  Transaction counts aren't known for block history loaded from a snapshot. If you start with `-checkblockindex` after loading a snapshot, the bitcoin daemon will core dump. The test suite does not check for this because all the snapshots have no non-coinbase transactions (all blocks prior to the snapshot are assumed to have `nTx = 1`).

  Recommend for backport to 26.x

ACKs for top commit:
  fjahr:
    utACK cdc6ac4
  achow101:
    ACK cdc6ac4
  pablomartin4btc:
    tACK cdc6ac4

Tree-SHA512: f7488a85cc29056e2ac443ce8f34aea4dfde6ba246efce82235d6a4dca2dca4344f07b93c93424b4addcb83e4cb2ae49a3ebb37d89840d42d2aeea35904cab04

Author: achow101

src/validation.cpp

@@ -4868,7 +4868,9 @@ void ChainstateManager::CheckBlockIndex()
                // For testing, allow transaction counts to be completely unset.
                || (pindex->nChainTx == 0 && pindex->nTx == 0)
                // For testing, allow this nChainTx to be unset if previous is also unset.
-               || (pindex->nChainTx == 0 && prev_chain_tx == 0 && pindex->pprev));
+               || (pindex->nChainTx == 0 && prev_chain_tx == 0 && pindex->pprev)
+               // Transaction counts prior to snapshot are unknown.
+               || pindex->IsAssumedValid());
 
         if (pindexFirstAssumeValid == nullptr && pindex->nStatus & BLOCK_ASSUMED_VALID) pindexFirstAssumeValid = pindex;
         if (pindexFirstInvalid == nullptr && pindex->nStatus & BLOCK_FAILED_VALID) pindexFirstInvalid = pindex;