[C++] ConvertColumnReader.TestConvertNumericToStringVariant fails when compiled with unsigned char

[----------] 6 tests from ConvertColumnReader
[ RUN ] ConvertColumnReader.betweenNumericWithoutOverflows
[ OK ] ConvertColumnReader.betweenNumericWithoutOverflows (7 ms)
[ RUN ] ConvertColumnReader.betweenNumricOverflows
[ OK ] ConvertColumnReader.betweenNumricOverflows (0 ms)
[ RUN ] ConvertColumnReader.booleanToString
[ OK ] ConvertColumnReader.booleanToString (0 ms)
[ RUN ] ConvertColumnReader.TestConvertNumericToStringVariant
/root/orc/c++/test/TestConvertColumnReader.cc:339: Failure
Expected equality of these values:
expected[1]
Which is: "254"
std::string(readC1.data[i], static_cast<size_t>(readC1.length[i]))
Which is: "-2"
2
/root/orc/c++/test/TestConvertColumnReader.cc:339: Failure
Expected equality of these values:
expected[1]
Which is: "252"
std::string(readC1.data[i], static_cast<size_t>(readC1.length[i]))
Which is: "-4"
4