fix: add vtxundo size validation in SpentIndex::WriteBlock

Match the same defensive check that AddressIndex::WriteBlock performs,
preventing potential out-of-bounds access on block.vtx if undo data is
inconsistent.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: UdjinM6

src/index/spentindex.cpp

@@ -78,6 +78,11 @@ bool SpentIndex::WriteBlock(const CBlock& block, const CBlockIndex* pindex)
 
     // Process each non-coinbase transaction
     // blockundo.vtxundo[i] corresponds to block.vtx[i+1] (coinbase is skipped in undo data)
+    if (blockundo.vtxundo.size() != block.vtx.size() - 1) {
+        return error("%s: Undo data size mismatch for block %s (expected %zu, got %zu)", __func__,
+                     pindex->GetBlockHash().ToString(), block.vtx.size() - 1, blockundo.vtxundo.size());
+    }
+
     for (size_t i = 0; i < blockundo.vtxundo.size(); i++) {
         const CTransactionRef& tx = block.vtx[i + 1]; // +1 to skip coinbase
         const CTxUndo& txundo = blockundo.vtxundo[i];