Fix compile issue related to std::optional only being in c++17

src/wallet/wallet.cpp

@@ -1203,7 +1203,7 @@ void CWallet::BuildWitnessCache(const CBlockIndex* pindex, bool witnessOnly)
             // so we stop updating their witnesses. This is safe to do because we know
             // we won't roll back more than WITNESS_CACHE_SIZE blocks, since it is a
             // number larger than the max reorg length
-            if (nd->spentHeight.has_value() && nd->spentHeight.value() + WITNESS_CACHE_SIZE < indexHeight) {
+            if (nd->spentHeight > 0 && nd->spentHeight + WITNESS_CACHE_SIZE < indexHeight) {
                 LogPrintf("%s: pruning witnesses for %s at indexHeight=%d with spentHeight=%d\n", __func__, nullifier.GetHex().c_str(), indexHeight, nd->spentHeight);
                 nd->witnesses.clear();
                 nd->witnessHeight = -1;

src/wallet/wallet.h

@@ -295,7 +295,7 @@ public:
      * CWallet::BuildWitnessCache and CWallet::DecrementNoteWitnesses.
      */
 
-    SaplingNoteData() : witnessHeight {-1}, nullifier(), spentHeight() { }
+    SaplingNoteData() : witnessHeight {-1}, nullifier(), spentHeight(-1) { }
     SaplingNoteData(libzcash::SaplingIncomingViewingKey ivk) : ivk {ivk}, witnessHeight {-1}, nullifier() { }
     SaplingNoteData(libzcash::SaplingIncomingViewingKey ivk, uint256 n) : ivk {ivk}, witnessHeight {-1}, nullifier(n) { }
 
@@ -327,7 +327,7 @@ public:
      * witnesses will continue to be updated, which is only a performance
      * rather than a correctness issue).
      */
-    std::optional<int> spentHeight;
+    int spentHeight;
 
     ADD_SERIALIZE_METHODS;