Fix crash on shutdown if wxConfig couldn't be saved

If saving wxConfig to the file in wxAppConsoleBase::CleanUp() failed,
wxLog messages were logged into a wxLogGui target which was recreated
because wxTheApp is still non-null when CleanUp() is called. However it
is destroyed soon afterwards and showing messages from wxLogGui without
wxTheApp leads to crashes, at least in wxGTK.

To avoid this, we could set wxTheApp to null before calling CleanUp(),
but it's not clear if there is no cleanup code (or something called from
it) which doesn't rely on wxTheApp existence, so instead just flush, and
reset, the log target after calling CleanUp() but before destroying the
application object.

This fixes the crash which could be easily reproduced simply by making
the file used by wxFileConfig not-writeable under Linux.

src/common/init.cpp

@@ -488,14 +488,18 @@ static void DoCommonPostCleanup()
 
 void wxEntryCleanup()
 {
-    DoCommonPreCleanup();
-
-
     // delete the application object
     if ( wxTheApp )
     {
         wxTheApp->CleanUp();
+    }
 
+    // It's important to call this after wxApp::CleanUp() as it can log some
+    // messages that will be flushed inside DoCommonPreCleanup().
+    DoCommonPreCleanup();
+
+    if ( wxTheApp )
+    {
         // reset the global pointer to it to nullptr before destroying it as in
         // some circumstances this can result in executing the code using
         // wxTheApp and using half-destroyed object is no good