avoid lambda to simplify source code

dbc_decoder.cpp

@@ -57,11 +57,12 @@ DecodedFrameValue DbcDecoder::Decode(const DecodeDatabase &database,
         else
             decoded.rawValue = unsignedValue;
 
-        decoded.physicalValue =
-                std::visit([&signal](auto rawValue) {
-                    return (static_cast<double>(rawValue) * signal.factor) + signal.offset;
-                },
-                           decoded.rawValue);
+        double numericValue;
+        if (const auto *signedValue = std::get_if<std::int64_t>(&decoded.rawValue))
+            numericValue = static_cast<double>(*signedValue);
+        else
+            numericValue = static_cast<double>(std::get<std::uint64_t>(decoded.rawValue));
+        decoded.physicalValue = numericValue * signal.factor + signal.offset;
         decoded.valid = true;
 
         result.signals.push_back(decoded);