LU-14535 utils: fix FORWARD_NULL issue from Coverity

LU-14535 utils: fix FORWARD_NULL issue from Coverity

Fixing the possible NULL pointer issued reported from Coverity

   case 'e':
CID 424708:    (FORWARD_NULL)
Passing null pointer "optarg" to "strtoul", which dereferences it.
      end_qid = strtoul(optarg, NULL, 0);
      break;

CoverityID: 424708 ("FORWARD NULL")

Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: Idfb5cb4c6fe63ec08dd9048742f3f280b125eb8a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54827
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>