LU-13577 wbc: theoretical link with missing destination

LU-13577 wbc: theoretical link with missing destination

While unlikely it would be possible so return -EINVAL here

CoverityID: 425353 ("Null pointer dereferences (FORWARD NULL)")

Fixes: 668dfb53de ("LU-13577 wbc: reimplement mkdir() by using intent lock")
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I2a41d1a37820a3bc7b06dff42a4cc09386a88820
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55016
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>