wallet: Ignore MarkConflict if block hash is not known

If number of conflict confirms cannot be determined, this means
that the block is still unknown or not yet part of the main chain,
for example during a reindex. Do nothing in that case,
instead of crash with an assertion.

Fixes bitcoin#7234.

src/wallet/wallet.cpp

@@ -846,14 +846,19 @@ void CWallet::MarkConflicted(const uint256& hashBlock, const uint256& hashTx)
 {
     LOCK2(cs_main, cs_wallet);
 
-    CBlockIndex* pindex;
-    assert(mapBlockIndex.count(hashBlock));
-    pindex = mapBlockIndex[hashBlock];
     int conflictconfirms = 0;
-    if (chainActive.Contains(pindex)) {
-        conflictconfirms = -(chainActive.Height() - pindex->nHeight + 1);
+    if (mapBlockIndex.count(hashBlock)) {
+        CBlockIndex* pindex = mapBlockIndex[hashBlock];
+        if (chainActive.Contains(pindex)) {
+            conflictconfirms = -(chainActive.Height() - pindex->nHeight + 1);
+        }
     }
-    assert(conflictconfirms < 0);
+    // If number of conflict confirms cannot be determined, this means
+    // that the block is still unknown or not yet part of the main chain,
+    // for example when loading the wallet during a reindex. Do nothing in that
+    // case.
+    if (conflictconfirms >= 0)
+        return;
 
     // Do not flush the wallet here for performance reasons
     CWalletDB walletdb(strWalletFile, "r+", false);