Fix unbound local error when using the fsoids.py script (#295)

Co-authored-by: ale-rt <alessandro.pisa@gmail.com>

CHANGES.rst

@@ -5,7 +5,8 @@
 5.6.1 (unreleased)
 ==================
 
-- Nothing changed yet.
+- Fix UnboundLocalError when running fsoids.py script.
+  See `issue 268 <https://github.com/zopefoundation/ZODB/issues/285>`_.
 
 
 5.6.0 (2020-06-11)

src/ZODB/serialize.py

@@ -699,7 +699,7 @@ def get_refs(a_pickle):
         if isinstance(reference, tuple):
             oid, klass = reference
         elif isinstance(reference, (bytes, str)):
-            data, klass = reference, None
+            oid, klass = reference, None
         else:
             assert isinstance(reference, list)
             continue