s3:loadparm: fix NULL pointer dereference in volume_label()

s3:loadparm: fix NULL pointer dereference in volume_label()

volume_label() calls lp_servicename() as a fallback when lp_volume()
returns an empty string. lp_servicename() is a FN_LOCAL_SUBSTITUTED_STRING
that falls back to sDefault.szService when the service is invalid. Since
sDefault.szService is initialized to NULL and is never set by
init_globals(), the substitution returns NULL, and the subsequent
strlen() call crashes with a segmentation fault.

Add a NULL guard so volume_label() returns an empty string instead
of crashing.

Remove knownfail.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14978

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: David Mulder <dmulder@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Apr 15 00:07:12 UTC 2026 on atb-devel-224