_weakref.__init__() doesn't catch errors correctly. Example:
--------------------- 8< -------------------------
from gc import collect
import _weakref

class FuzzingUserClass:
    pass

obj = _weakref.ref(FuzzingUserClass)

# Exception not raised??
obj.__init__(
    0,
    0,
    0,
)

# Exception catched here??
collect()
--------------------- 8< -------------------------

Attached patch fix the bug for py3k branch: return -1 on error 
(instead of 1).