pygpo: Fix a talloc_tos() leak in py_gpo_get_unix_path

pygpo: Fix a talloc_tos() leak in py_gpo_get_unix_path

cache_path() implicitly puts its result on talloc_tos(). As in
py_gpo_get_unix_path the talloc_stackframe() is only created after the
cache_path() call, we leak the result of cache_path() on
talloc_tos() (which might or might not exist).

This converts the function to the pattern used elsewhere: Create the
stackframe as the very first action and remove it as the very last
action in the function.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>