Attempting to call onMovePiece callback even when it isn't defined

[jniemann66]

In chessdiagram.js, the introduction 'allowMoves' was a good idea.
However, there should be an additional safeguard against the case where allowMoves is true, but onMovePiece is undefined.

I propose the following change to Chessdiagram.js, line 64:

_onMovePiece(pieceType, from, to) {
	if (this.props.allowMoves && this.props.onMovePiece) {
		this.props.onMovePiece(pieceType, from, to);
	}
}

[jniemann66]

fixed in f4e480c