LU-16097 quota: fix Null pointer dereference

LU-16097 quota: fix Null pointer dereference

The "qbody" should be checked whether it is NULL or not.

CoverityID: 410242 ("Dereference after null check")

Fixes: 57ac32a2 ("LU-16097 quota: release preacquired quota when over limits")
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: Idab61f3ebac24307c6d5db0d42429914858d21cb
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53330
Reviewed-by: Sergey Cheremencev <scherementsev@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>