[gjs] object: Fix dangling pointer crash

[Philip Chimento <pchimento src gnome org>]

commit 6a81382482bf84598b1dc1ed3990df65b041f2f3
Author: Jan Alexander Steffens (heftig) <jan steffens gmail com>
Date:   Sun Mar 1 19:53:10 2020 -0800

    object: Fix dangling pointer crash
    
    The linked list is used by update_heap_wrapper_weak_pointers() which is
    responsible for clearing weak pointers to the ObjectInstance wrapper
    that become dangling.
    
    There's a chance the GC sweeps the wrapper between the call to
    gobj_dispose_notify() and the ObjectInstance being destroyed. The GC
    will call the weak pointer callback when it does this.
    
    If the ObjectInstance is missing from the list at this point (because of
    the unlink()) then the weak pointer will be missed and become dangling.
    Afterwards, when the ObjectInstance is being destroyed, the Heap
    destructor will trigger the write pre-barrier, causing a crash.
    
    Closes: #301

 gi/object.cpp | 1 -
 1 file changed, 1 deletion(-)
---
diff --git a/gi/object.cpp b/gi/object.cpp
index 8c136267..7bc942b9 100644
--- a/gi/object.cpp
+++ b/gi/object.cpp
@@ -1071,7 +1071,6 @@ void
 ObjectInstance::gobj_dispose_notify(void)
 {
     m_gobj_disposed = true;
-    unlink();
 }
 
 void ObjectInstance::iterate_wrapped_gobjects(