Fixed a bug that caused crashes when running multiple consecutive games.

in BWAPI::GameImpl::onGameStart: players with a null force is assigned the neutral force, but the value wasn't reset between games. This could result in the next game having two different neutral forces and more than 5 forces in total, which crashes BWAPIClient.
Merge pull request #521 from tscmoo/dev

Fixed a bug that caused crashes when running multiple consecutive games.