make sure "current" is initialized

Also, a nit: fix indentation of 1 line in AllPlotWindow.h.

src/AllPlotWindow.cpp

@@ -36,7 +36,7 @@
 #include <qwt_arrow_button.h>
 
 AllPlotWindow::AllPlotWindow(MainWindow *mainWindow) :
-    QWidget(mainWindow), mainWindow(mainWindow)
+    QWidget(mainWindow), mainWindow(mainWindow), current(NULL)
 {
     QVBoxLayout *vlayout = new QVBoxLayout;