_setstate_noncurrent(): New assert. Hoping to get closer to the true

cause of the:

  TypeError: unpack() argument 2 must be string or read-only buffer, not None

failure often seen in overnight testrunner reports now, coming from the
checkConcurrentUpdatesInVersions test.  If the hypothesis is correct,
we'll see a failure of the new

        assert end is not None

instead.

src/ZODB/Connection.py

@@ -13,7 +13,7 @@
 ##############################################################################
 """Database connection support
 
-$Id: Connection.py,v 1.153 2004/04/16 20:26:16 jeremy Exp $"""
+$Id: Connection.py,v 1.154 2004/04/19 20:24:02 tim_one Exp $"""
 
 import logging
 import sys
@@ -875,6 +875,7 @@ class Connection(ExportImport, object):
         # been modified at txn_time.
 
         assert start < self._txn_time, (u64(start), u64(self._txn_time))
+        assert end is not None
         assert self._txn_time <= end, (u64(self._txn_time), u64(end))
         self._reader.setGhostState(obj, data)
         obj._p_serial = start