Initialize member variables

It's always good practice.

lib/History.cpp

@@ -90,7 +90,8 @@ FIXME: There is noticeable decrease in speed, also. Perhaps,
 HistoryFile::HistoryFile()
   : ion(-1),
     length(0),
-    fileMap(nullptr)
+    fileMap(nullptr),
+    readWriteBalance(0)
 {
   if (tmpFile.open())
   {

lib/ScreenWindow.cpp

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