Fix segfault in bucket_repr() on Python 3

Fixes #106.

The bug was in not checking the return value of
PyUnicode_AsLatin1String() and passing a NULL to PyOS_snprintf().

The fix is to use PyUnicode_FromFormat() and avoid all the hassle of
manual encoding/sprintf/decoding and even calling repr().

BTrees/BucketTemplate.c

@@ -1814,51 +1814,40 @@ static PyObject *
 bucket_repr(Bucket *self)
 {
     PyObject *i, *r;
-#ifdef PY3K
-    PyObject *rb;
-#endif
+#ifndef PY3K
     char repr[10000];
     int rv;
+#endif
 
     i = bucket_items(self, NULL, NULL);
     if (!i)
     {
         return NULL;
     }
+#ifdef PY3K
+    r = PyUnicode_FromFormat("%s(%R)", Py_TYPE(self)->tp_name, i);
+    Py_DECREF(i);
+    return r;
+#else
     r = PyObject_Repr(i);
     Py_DECREF(i);
     if (!r)
     {
         return NULL;
     }
-#ifdef PY3K
-    rb = PyUnicode_AsLatin1String(r);
-    rv = PyOS_snprintf(repr, sizeof(repr),
-                       "%s(%s)", Py_TYPE(self)->tp_name,
-                       PyBytes_AsString(rb));
-    Py_DECREF(rb);
-#else
     rv = PyOS_snprintf(repr, sizeof(repr),
                        "%s(%s)", Py_TYPE(self)->tp_name,
                        PyBytes_AS_STRING(r));
-#endif
     if (rv > 0 && (size_t)rv < sizeof(repr))
     {
         Py_DECREF(r);
-#ifdef PY3K
-        return PyUnicode_DecodeLatin1(repr, strlen(repr), "surrogateescape");
-#else
         return PyBytes_FromStringAndSize(repr, strlen(repr));
-#endif
     }
     else
     {
         /* The static buffer wasn't big enough */
         int size;
         PyObject *s;
-#ifdef PY3K
-        PyObject *result;
-#endif
         /* 3 for the parens and the null byte */
         size = strlen(Py_TYPE(self)->tp_name) + PyBytes_GET_SIZE(r) + 3;
         s = PyBytes_FromStringAndSize(NULL, size);
@@ -1869,14 +1858,9 @@ bucket_repr(Bucket *self)
         PyOS_snprintf(PyBytes_AS_STRING(s), size,
                     "%s(%s)", Py_TYPE(self)->tp_name, PyBytes_AS_STRING(r));
         Py_DECREF(r);
-#ifdef PY3K
-        result = PyUnicode_FromEncodedObject(s, "latin1", "surrogateescape");
-        Py_DECREF(s);
-        return result;
-#else
         return s;
-#endif
     }
+#endif
 }
 
 static PyTypeObject BucketType = {