[ZODB4] Fix unbound local error when using the fsoids.py script (#295)

( This is ZODB4 backport of upstream commit fc4c86e6 )
Co-authored-by: ale-rt <alessandro.pisa@gmail.com>

CHANGES.rst

@@ -5,6 +5,8 @@
 4.4.6 (unreleased)
 ==================
 
+- Fix UnboundLocalError when running fsoids.py script.
+  See `issue 268 <https://github.com/zopefoundation/ZODB/issues/285>`_.
 - Fixed: When opening FileStorages in read-only mode, non-existent
   files were silently created.  Creating a read-only file-storage
   against a non-existent file errors.

src/ZODB/serialize.py

@@ -688,7 +688,7 @@ 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