wallet: refactor: avoid duplicate lookup on mapValue["timesmart"]

Also, use a named cast for converting the atoi64() result into an
unsigned int type.

src/wallet/transaction.h

@@ -217,7 +217,8 @@ public:
 
         const auto it_op = mapValue.find("n");
         nOrderPos = (it_op != mapValue.end()) ? atoi64(it_op->second) : -1;
-        nTimeSmart = mapValue.count("timesmart") ? (unsigned int)atoi64(mapValue["timesmart"]) : 0;
+        const auto it_ts = mapValue.find("timesmart");
+        nTimeSmart = (it_ts != mapValue.end()) ? static_cast<unsigned int>(atoi64(it_ts->second)) : 0;
 
         mapValue.erase("fromaccount");
         mapValue.erase("spent");