s3: winbind: Fix crash when invoking winbind idmap scripts.

s3: winbind: Fix crash when invoking winbind idmap scripts.

Previously the private context was caching a pointer to
a string returned from lp_XXX(). This string can change
on config file reload. Ensure the string is talloc_strup'ed
onto the owning context instead.

Reported by Heinrich Mislik <Heinrich.Mislik@univie.ac.at>

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13956

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
(cherry picked from commit a1f95ba5db6fc017fad35377fbf76c048f2dd8ab)