z.i.adapters.LookupBase.lookup1 inconsistent between Python and C implementations

The reference implementation for lookup1 consistently returns the '_default'
if the value returned by 'lookup' is None; the C version only does so on the
first time through: after that, it returns the (cached) None. E.g.:

    from zope.interface.adapters import LookupBase
    class Lookup(LookupBase):
        def _uncached_lookup(self, required, provided, name):
            return None
    lb = Lookup()
    _default = object()
    found = lb.lookup1('A', 'B', 'C', _default)
    assert(found is _default) # OK both in C and Python
    found = lb.lookup1('A', 'B', 'C', _default)
    assert(found is _default) # OK in Python, fails in C

I will attaching a patch which fixes this problem. The patch, as well as
a testcase demonstrating the bug, are also in my branch:

  lp:~tseaver/zope.interface/bettter_unittests