fix analysis of entry to be a weakref (typo introduced in d96dfd)

Originally reported in Debian http://bugs.debian.org/cgi-bin/bugreport.cgi\?bug\=692313
Lead to crash with python-dbg:

  python-dbg: ../Modules/gcmodule.c:366: visit_decref: Assertion 'gc->gc.gc_refs != 0' failed