Set Board::turn_color to valid value before use
Set Board::turn_color to valid value before use

Previous implementation called Board::set_turn(), which would only set a
valid value for the input color being BLACK, leaving a garbage value
otherwise.

MarkZH/Genetic_Chess,MarkZH/Genetic_Chess,MarkZH/Genetic_Chess,MarkZH/Genetic_Chess,MarkZH/Genetic_Chess