Fix nss_nisplus build with mainline GCC (bug 20978)

[Joseph Myers]

On Tue, 20 Dec 2016, Joseph Myers wrote:

> x86_64.  However, I have not tried any actual substantive nisplus
> testing, do not have an environment for such testing, and do not know
> whether it is possible that strlen (name) or tablename_len might be
> large so that the VLA for buf is actually a security issue.  However,
> if it is a security issue, there are plenty of other similar instances
> in the nisplus code (that haven't been hidden by a bogus comparison
> with NULL) - and nis_table.c:__create_ib_request uses strdupa on the
> string passed to nis_list, so a local fix in the caller wouldn't
> suffice anyway (and maybe if there are issues that means a separate
> bug or bugs should be filed to track them).  (Calls to strdupa and

Bug 20987 now filed for this group of apparently unbounded NIS+ stack 
allocations.  I hope this build fix patch can be considered separately 
from any fixes needed for those stack allocations.

-- 
Joseph S. Myers
joseph@codesourcery.com