initialize wckey to NULL for job update RPC.

src/api/init_msg.c

@@ -136,6 +136,7 @@ void slurm_init_job_desc_msg(job_desc_msg_t * job_desc_msg)
 	job_desc_msg->task_dist		= (uint16_t) NO_VAL;
 	job_desc_msg->time_limit	= NO_VAL;
 	job_desc_msg->user_id		= NO_VAL;
+	job_desc_msg->wckey             = NULL;
 	job_desc_msg->work_dir		= NULL;
 }