Avoid -Werror,-Wnull-dereference

Change-Id: Ibfc0111a61817b7cf8c5b60d3f9563fd3cf05c5c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/183011
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>
Tested-by: Jenkins

desktop/test/deployment/crashextension/crashextension.cxx

@@ -127,7 +127,8 @@ Provider::queryDispatches(css::uno::Sequence<css::frame::DispatchDescriptor> con
 void Provider::dispatch(css::util::URL const&, css::uno::Sequence<css::beans::PropertyValue> const&)
 {
     // Crash LibreOffice
-    *((char*)NULL) = 0;
+    char volatile* p = nullptr;
+    *p = 0;
 }
 
 cppu::ImplementationEntry const services[]