Small fixes.

src/effect_matrix.cpp

@@ -154,7 +154,7 @@ RandomMatrixEffect::RandomMatrixEffect() {
 RainbowMatrixEffect::RainbowMatrixEffect() {
 	// No need to initialize _columns, because that will have been done by ctor of MatrixEffect.
 	for (int i=0; i<window->width; i++) {
-		delete columns[i];
+		delete _columns[i];
 		_columns[i] = new RainbowMatrixEffectColumn(window, MatrixEffectColumn::DIR_SOUTH);
 	}
 }