nss-idmap: use right group list pointer after sss_get_ex()
If the initial array is too small it will be reallocated during
sss_get_ex() and the pointer might change and the initial memory area
should not be used anymore.
Related to https://pagure.io/SSSD/sssd/issue/3715
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>