Make Callable not crash on call when the object has been freed

Also add a GDScript test for this case.

modules/gdscript/tests/scripts/runtime/errors/callable_call_after_free_object.gd

@@ -0,0 +1,5 @@
+func test():
+	var node := Node.new()
+	var inside_tree = node.is_inside_tree
+	node.free()
+	inside_tree.call()

modules/gdscript/tests/scripts/runtime/errors/callable_call_after_free_object.out

@@ -0,0 +1,6 @@
+GDTEST_RUNTIME_ERROR
+>> SCRIPT ERROR
+>> on function: test()
+>> runtime/errors/callable_call_after_free_object.gd
+>> 5
+>> Attempt to call function 'null::is_inside_tree (Callable)' on a null instance.