checker: fix fntype var marked as auto heap (#22290)

vlib/v/checker/checker.v

@@ -4376,6 +4376,13 @@ fn (mut c Checker) mark_as_referenced(mut node ast.Expr, as_interface bool) {
 							}
 						}
 						.sum_type, .interface_ {}
+						.function {
+							if type_sym.info is ast.FnType {
+								if type_sym.info.is_anon {
+									node.obj.is_auto_heap = true
+								}
+							}
+						}
 						else {
 							node.obj.is_auto_heap = true
 						}

vlib/v/tests/pointers/ref_callback_test.v

@@ -0,0 +1,20 @@
+module main
+
+struct Struct_voidptr {
+	func voidptr
+}
+
+fn function() string {
+	return 'Function!'
+}
+
+fn test_main() {
+	fun := function
+
+	sct := Struct_voidptr{
+		func: &fun
+	}
+
+	assert fun() == 'Function!'
+	assert sct.func == &fun
+}