Fix QCFType::constructFromGet() crash when passed a nullptr

Change-Id: I83cbbb47af8580fa67cbc75fee07bc1e123895eb
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qcore_mac_p.h

@@ -111,6 +111,7 @@ public:
     template <typename X> X as() const { return reinterpret_cast<X>(type); }
     static QCFType constructFromGet(const T &t)
     {
+        if (t)
             CFRetain(t);
         return QCFType<T>(t);
     }