wallet: Check last block and conflict height are valid in MarkConflicted

MarkConflicted calculates conflict confirmations incorrectly when both
the last block processed height and the conflicting height are negative
(i.e. uninitialized). If either are negative, we should not be marking
conflicts and should exit early.

src/wallet/wallet.cpp

@@ -1339,11 +1339,14 @@ void CWallet::MarkConflicted(const uint256& hashBlock, int conflicting_height, c
 {
     LOCK(cs_wallet);
 
-    int conflictconfirms = (m_last_block_processed_height - conflicting_height + 1) * -1;
     // 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 (m_last_block_processed_height < 0 || conflicting_height < 0) {
+        return;
+    }
+    int conflictconfirms = (m_last_block_processed_height - conflicting_height + 1) * -1;
     if (conflictconfirms >= 0)
         return;