SceneGraph: fix assertion expression.

We should have used == instead of =, the counter is set to that value
later anyway, so this didn't break anything... but the assert was
useless. Hopefully it won't fire each time now :-)

src/SceneGraph/Object.hpp

@@ -282,7 +282,7 @@ template<class Transformation> std::vector<typename Transformation::DataType> Ob
     for(auto i: jointObjects) {
         /* All not-already cleaned objects (...duplicate occurences) should
            have joint mark */
-        CORRADE_INTERNAL_ASSERT(i->counter = 0xFFFFu || i->flags & Flag::Joint);
+        CORRADE_INTERNAL_ASSERT(i->counter == 0xFFFFu || i->flags & Flag::Joint);
         i->flags &= ~Flag::Joint;
         i->counter = 0xFFFFu;
     }