Do not crash when checking whether an instance type is of a given fundamental.

Not all instances have a TypeNode associated (e.g. GstEvent), so lets check if node is available
before trying to use it.

This crash can be easily reproduced by creating an event with gst_event_new_eos and using
G_IS_OBJECT on the event instance.

https://bugzilla.gnome.org/show_bug.cgi?id=733982
gtype: Fast-path for g_type_is_a

Do I really need to explain why ?

https://bugzilla.gnome.org/show_bug.cgi?id=732085