jqueue: Fix error when WaitForJobChange gets invalid ID

When JobQueue.WaitForJobChange gets an invalid or no longer existing job ID it
tries to return job_info and log_entries, both of which aren't defined yet.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

lib/jqueue.py

@@ -1101,6 +1101,10 @@ class JobQueue(object):
 
     """
     logging.debug("Waiting for changes in job %s", job_id)
+
+    job_info = None
+    log_entries = None
+
     end_time = time.time() + timeout
     while True:
       delta_time = end_time - time.time()
@@ -1142,7 +1146,10 @@ class JobQueue(object):
 
     logging.debug("Job %s changed", job_id)
 
-    return (job_info, log_entries)
+    if job_info is None and log_entries is None:
+      return None
+    else:
+      return (job_info, log_entries)
 
   @utils.LockedMethod
   @_RequireOpenQueue