Don't dereference NULL output pointer in wxTextMeasure.

Check that pointers are non-NULL before assigning 0 to them.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73016 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/textmeasure.cpp

@@ -116,7 +116,10 @@ void wxTextMeasure::DoGetTextExtent(const wxString& string,
 {
     if ( !m_context )
     {
-        *width =
+        if ( width )
+            *width = 0;
+
+        if ( height )
             *height = 0;
         return;
     }