Fix exception when creating a wxGauge.

Platforms that don't make use of wxGaugeBase::Create when creating a
wxGauge (such as GTK and MSW) access the uninitialised
m_appProgressIndicator. Fix by initialising m_appProgressIndicator to NULL
in the wxGaugeBase constructor. Regression since r78499.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78512 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gauge.h

@@ -51,7 +51,9 @@ class WXDLLIMPEXP_FWD_CORE wxAppProgressIndicator;
 class WXDLLIMPEXP_CORE wxGaugeBase : public wxControl
 {
 public:
-    wxGaugeBase() { m_rangeMax = m_gaugePos = 0; }
+    wxGaugeBase() : m_rangeMax(0), m_gaugePos(0),
+        m_appProgressIndicator(NULL) { }
+
     virtual ~wxGaugeBase();
 
     bool Create(wxWindow *parent,