Initialize member variables

It's always good practice.

lib/ScreenWindow.cpp

@@ -30,6 +30,7 @@ using namespace Konsole;
 
 ScreenWindow::ScreenWindow(QObject* parent)
     : QObject(parent)
+    , _screen(nullptr)
     , _windowBuffer(nullptr)
     , _windowBufferSize(0)
     , _bufferNeedsUpdate(true)