Merge bitcoin#13657: wallet: assert to ensure accuracy of CMerkleTx::…

…GetBlocksToMaturity

93de289 wallet: assert to ensure accuracy of CMerkleTx::GetBlocksToMaturity (Ben Woosley)

Pull request description:

  According to my understanding, it should not be possible for coinbase
  transactions to be conflicting, thus it should not be possible for
  GetDepthInMainChain to return a negative result. If it did, this would
  also result in innacurate results for GetBlocksToMaturity due to the
  math therein. asserting ensures accuracy.

Tree-SHA512: 8e71c26f09fe457cfb00c362ca27066f7f018ea2af1f395090fdc7fd9f5964b76f4317c23f7a4923776f00087558511da5c1c368095be39fb1bacc614a93c32f

src/wallet/wallet.cpp

@@ -5607,7 +5607,9 @@ int CMerkleTx::GetBlocksToMaturity() const
 {
     if (!IsCoinBase())
         return 0;
-    return std::max(0, (COINBASE_MATURITY+1) - GetDepthInMainChain());
+    int chain_depth = GetDepthInMainChain();
+    assert(chain_depth >= 0); // coinbase tx should not be conflicted
+    return std::max(0, (COINBASE_MATURITY+1) - chain_depth);
 }