Allocating a new weakref object can cause existing weakref objects for

Allocating a new weakref object can cause existing weakref objects for
the same object to be collected by the cyclic GC support if they are
only referenced by a cycle.  If the weakref being collected was one of
the weakrefs without callbacks, some local variables for the
constructor became invalid and have to be re-computed.

The test caused a segfault under a debug build without the fix applied.