Fix crash when using default wxDataViewEvent ctor

Check the value of the wxDataViewCtrl pointer parameter in Init() instead
of unconditionally calling GetModel() on it. The default ctor of
wxDataViewEvent calls Init() with a null pointer.

This fixes the failing GUI tests with the XP buildbot slave.

Regression since 9829446755.

Also see #12649.

src/common/datavcmn.cpp

@@ -1612,7 +1612,7 @@ void wxDataViewEvent::Init(wxDataViewCtrlBase* dvc,
 {
     m_item = item;
     m_col = column ? column->GetModelColumn() : -1;
-    m_model = dvc->GetModel();
+    m_model = dvc ? dvc->GetModel() : NULL;
     m_column = column;
     m_pos = wxDefaultPosition;
     m_cacheFrom = 0;