Fix weakrefs on large objects

We would keep on adding the same object to the weakrefs list,
which would eventually trigger a collection, and cause all sorts
of weird things to happen.

src/gc/heap.cpp

@@ -95,12 +95,13 @@ inline void sweepList(ListT* head, std::list<Box*, StlCompatAllocator<Box*>>& we
             cur = cur->next;
         } else {
             if (_doFree(al, &weakly_referenced)) {
-
                 removeFromLL(cur);
 
                 auto to_free = cur;
                 cur = cur->next;
                 free_func(to_free);
+            } else {
+                cur = cur->next;
             }
         }
     }

test/tests/weakref4.py

 import weakref
-import gc
 
 def doStuff():
   def meth():
@@ -16,5 +15,18 @@ def fact(n):
 
 w = doStuff()
 print fact(10) # try to clear some memory
+
+# Try creating a large object to make sure we can handle them:
+def f():
+    class C(object):
+        # Adding a __slots__ directive increases the size of the type object:
+        __slots__ = ['a' + str(i) for i in xrange(1000)]
+    return weakref.ref(C)
+
+
+r = f()
+
+import gc
 gc.collect()
-print w()
+assert r() is None, "object was not collected"
+assert w() is None, "object was not collected"