interfaces: Change getUnspentOutput return type to avoid multiprocess segfault

Coin serialize method segfaults if IsSpent condition is true. This caused
multiprocess code to segfault when serializing the Coin& output argument to of
the Node::getUnspentOutput method if the coin was not found. Segfault could be
triggered by double clicking and viewing transaction details in the GUI
transaction list.

Fix this by replacing Coin& output argument with optional<Coin> return value to
avoid trying to serializing spent coins.

src/qt/transactiondesc.cpp

@@ -360,12 +360,10 @@ QString TransactionDesc::toHTML(interfaces::Node& node, interfaces::Wallet& wall
         {
             COutPoint prevout = txin.prevout;
 
-            Coin prev;
-            if(node.getUnspentOutput(prevout, prev))
-            {
+            if (auto prev{node.getUnspentOutput(prevout)}) {
                 {
                     strHTML += "<li>";
-                    const CTxOut &vout = prev.out;
+                    const CTxOut& vout = prev->out;
                     CTxDestination address;
                     if (ExtractDestination(vout.scriptPubKey, address))
                     {