qfontdatabase.cpp uses platformNativeInterface() unconditionally, leading to segfault

Line 536 of qfontdatabase.cpp is

QGuiApplication::platformNativeInterface()->nativeResourceForScreen("hintstyle", QGuiApplication::primaryScreen());

If a QPA plugin implements services() but does not implement nativeInterface() and the desktop name returned by serives is GNOME or UNITY Qt will crash.

The return value of platformNativeInterface() should be checked before being used.