valueflow.cpp: avoid unnecessary copy in valueFlowAfterAssign()

Author: firewave

lib/valueflow.cpp

@@ -4125,7 +4125,7 @@ static void valueFlowAfterAssign(TokenList &tokenlist,
                         continue;
                     ids.insert(value.tokvalue->exprId());
                 }
-                for (ValueFlow::Value value : values) {
+                for (ValueFlow::Value& value : values) {
                     if (!value.isSymbolicValue())
                         continue;
                     const Token* expr = value.tokvalue;