fix uninit valgrind error for base ctor in diamond inheritance
fix uninit valgrind error for base ctor in diamond inheritance

abutcher-gh/cereal,jkhoogland/cereal,fawkes74/cereal,USCiLab/cereal,USCiLab/cereal,abutcher-gh/cereal,adasworks/cereal,adasworks/cereal,jkhoogland/cereal,fawkes74/cereal