Fixed _pickle.Unpickler to handle empty persistent IDs correctly.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index bc3b406..d862ae8 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -4665,7 +4665,7 @@
     if (self->pers_func) {
         if ((len = _Unpickler_Readline(self, &s)) < 0)
             return -1;
-        if (len < 2)
+        if (len < 1)
             return bad_readline();
 
         pid = PyBytes_FromStringAndSize(s, len - 1);