Silence loplugin:unreffun with --disable-assert-always-abort

Change-Id: I1b5a7a6aa2436aa5b8297d5de172035cff3f1306
Reviewed-on: https://gerrit.libreoffice.org/47318Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

sdext/source/presenter/PresenterTimer.cxx

@@ -96,7 +96,9 @@ public:
         const TimeValue& rTimeValue);
 
     static void NotifyTermination();
+#if !defined NDEBUG
     static bool HasInstance() { return mpInstance != nullptr; }
+#endif
 
 private:
     static std::shared_ptr<TimerScheduler> mpInstance;