Fix p_secstodate overflow handling (bug 22463)

[Joseph Myers]

On Wed, 22 Nov 2017, Florian Weimer wrote:

> I think we can change the function to always return NULL (or abort the process
> with an error message) and turn it into into a compat symbol.  I very much
> doubt that there are any users at all because the function does not implement

Well, there are two calls in ns_print.c, so you can't eliminate the 
function without doing something with those calls.

-- 
Joseph S. Myers
joseph@codesourcery.com