log: Avoid broken SELECTCOINS log

Before this patch, the log might be corrupted by other threads logging
at the same time. For example, another RPC thread:

[httpworker.1] [default wallet] keypool reserve 1296
[httpworker.1] SelectCoins() best subset: Received a POST request for / from 127.0.0.1:53732
[httpworker.3] ThreadRPCServer method=getnetworkinfo user=__cookie__
[httpworker.1] 0.78125 0.1953125 0.02441406 0.00610351 0.00305175 0.00152587 total 1.01025417

Author: MarcoFalke

src/wallet/coinselection.cpp

@@ -279,13 +279,13 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
         }
 
         if (LogAcceptCategory(BCLog::SELECTCOINS)) {
-            LogPrint(BCLog::SELECTCOINS, "SelectCoins() best subset: "); /* Continued */
+            std::string log_message{"SelectCoins() best subset: "};
             for (unsigned int i = 0; i < applicable_groups.size(); i++) {
                 if (vfBest[i]) {
-                    LogPrint(BCLog::SELECTCOINS, "%s ", FormatMoney(applicable_groups[i].m_value)); /* Continued */
+                    log_message += strprintf("%s ", FormatMoney(applicable_groups[i].m_value));
                 }
             }
-            LogPrint(BCLog::SELECTCOINS, "total %s\n", FormatMoney(nBest));
+            LogPrint(BCLog::SELECTCOINS, "%stotal %s\n", log_message, FormatMoney(nBest));
         }
     }