LU-18793 gss: coverity issue in prepare_krb5_rfc4121_buffer

LU-18793 gss: coverity issue in prepare_krb5_rfc4121_buffer

Fix issue found by Coverity in prepare_krb5_rfc4121_buffer():

CoverityID 457079:    (RESOURCE_LEAK)
   Variable "derived_key" going out of scope leaks the storage
   "derived_key.data" points to.

Fixes: c7cf297687 ("LU-18256 gss: deprecate insecure enctypes")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1a2924555814ca6ce643b9e7cad217a7f6725765
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58371
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <adegremont@nvidia.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>