Fix AttributeError when sig is not a Signature. (#5297)

* Fix AttributeError when `sig` is not a Signature.

Fixes #5265.  Test is pending.

* Don't use 'isinstance' so that Mock objects are allowed in tests.

celery/canvas.py

@@ -1045,7 +1045,13 @@ def link(self, sig):
         return self.tasks[0].link(sig)
 
     def link_error(self, sig):
-        sig = sig.clone().set(immutable=True)
+        try:
+            sig = sig.clone().set(immutable=True)
+        except AttributeError:
+            # See issue #5265.  I don't use isinstance because current tests
+            # pass a Mock object as argument.
+            sig['immutable'] = True
+            sig = Signature.from_dict(sig)
         return self.tasks[0].link_error(sig)
 
     def _prepared(self, tasks, partial_args, group_id, root_id, app,