fix a segfault caused by stack overflow in handle_stageout

We met a segment fault when we submit a 'big' job to the cluster, such as 100 nodes * 25 ppn，when the submitter has no write permission to the current directory, then after the job is done, the job log can not be transferred back because of lack of permission, in which case the pbs_server segment fault.

Even though we meet this problem in torque 4.1.3, I noticed the latest code in upstream that the same problem may also exist.

The error is caused by stack overflow that the log_buf in handle_stageout() allocates a little big chunk of memory in stack which causes the stack overflow, this patch can fix this